Conventional spray painting, atomised coatings

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Conventional Air-operated Spraying

Conventional air-operated spray guns operate at very high pressure (typically 50 - 70 psi) which atomizes coatings to offer exceptionally fine finishes preferred by professionals and serious do-it-yourselfers for high quality paint jobs on automobiles, fleet vehicles and other metal surfaces.

Paint application by means of the spray is by no means a new process.This being the case, it is not surprising that there are still many misconceptions about it in the trade. Many people, for example, still regard the spray as primarily intended for the application of cellulose lacquers and enamels, and although it is certainly essential for the latter, it is equally capable of coating surfaces with oil paints, water paints, varnishes, and other forms of finish. Then again, it is often looked upon as a substitute for the brush; something which will make the traditional tool of the painter entirely obsolete. This, it may be said at once, is an entirely wrong conception, for not even the greatest spray enthusiast will deny that there are occasions when it is definitely uneconomical to use the spray on small surfaces such as window sashes, for instance, any advantage in the speed of application which the spray possesses will probably be more than counterbalanced by the time taken in masking the surrounding areas. The brush and the spray should therefore be regarded, not as rivals but as complementary to each other, each serving a useful purpose.

The value of the spray lies first in the greatly increased speed by which paint can be applied and in the beautifully smooth and even cutaway spray gun finish which can be obtained after only a minimum of experience. Another great advantage is the number of beautiful decorative effects which can be produced without difficulty-most of them far beyond the capacity of the brush hand.

Spray plants are now produced in various sizes, ranging from the small portable plant, which can be easily carried about from job to job, to the big plant, mounted on a truck, from which a battery of spray guns may be operated. The choice of a plant should be governed entirely by the class of work which the decorator expects to undertake, and if the spray is used intelligently, its initial cost will speedily be repaid by the economy in time and labour and other advantages which result from its use.

Principles Involved

To understand the art of spraying and to become an efficient operator, it is essential that the principles involved should be grasped. The success of the process depends upon the stream of paint or other type of finish being properly atomised, or, in other words, broken up into a fine mist. This state can only be obtained by two conditions working in harmony with each other-the flow of air from the pistol and the viscosity of the material which is being applied. If the force of the air is insufficient to break up the paint into fine particles, then, instead of an even film of paint being deposited upon the surface, blobs will be spurted out, and this will happen also if the paint is too stout or stodgy. It is essential, therefore, that experiments should be made to ascertain the correct balance of air and viscosity to produce satisfactory results. Different materials will require their own individual adjustments, but these will soon be determined with practice.

There are two systems adopted for obtaining atomisation. In one, the air is compressed and, when released, forces the material on to the surface at comparatively high pressure; in the other, the material is ejected on to the surface by means of a volume of air at relatively low pressure. Each has its advocates. For the high-pressure system it is claimed, for example, that finer atomisation is obtained; in the air-volume system, on the other hand, the degree of atomisation is not so high, but it is urged that the amount of spray dust which rebounds from the surface is considerably diminished, which is certainly an advantage when working in a confined space. In addition, there is less loss of the volatile constituents of the paint by evaporation; in consequence, the finish is deposited on the spray gun surface under treatment in a rather more wet condition, and this, to some extent, assists flow and adhesion. In the first system a pressure varying from about 50 to 70 lb. is used, whereas with the air-volume process, the pressure will be from about 5 to io lb. The air-volume system does not, however, lend itself quite so well to the application of certain types of finishes as does the high-pressure system.

It may here be remarked that some paint manufacturers indicate on the labels of the containers in which their product is supplied, or in the literature which accompanies it, the pressure at which it is recommended that the material is sprayed. Such information is valueless unless the type of spray gun and the viscosity of the finish are also indicated.

Spray plant consists of a number of units, among the most important of which is the compressor, the function of which is to supply an adequate volume of air at the right pressure. The air, once compressed, is stored in an air-storage tank, so that a reserve can be maintained and so that a steady supply of air can be provided without pulsations such as would occur if the air passed direct from the compressor to the spray gun From the decorator’s point of view, however, the gun is the unit which chiefly concerns him and it is not proposed to describe the mechanism of other parts of the equipment. To ensure maximum efficiency, they must be kept in good working order, but full instructions for doing this are provided with every outfit and these should be carefully studied before the plant is put into service.

The Spray Gun

The gun is a delicate tool and requires careful handling and attention:
it is an assembly of carefully balanced parts, and if any of these receive rough treatment or is neglected the smooth working of the instrument may be destroyed and partial replacement become necessary. On the other hand, normal care and attention will prolong the life of the gun almost indefinitely.

Spray guns vary in design according to their make, but if the diagram on page 6x of a sectional view of a spray gun is studied, it should not be difficult to understand the principles upon which the gun works.

The trigger controls a valve which allows the compressed air, which enters via the handle of the gun, to pass to the spray-head or cap. The paint or other material which is being applied may be in a container fixed on the top of the gun, in which case it flows to the point of ejection by the force of gravity. Alternatively, the container may be attached underneath the gun, the paint being drawn up by suction; or again, the material may be stored in what are known as pressure-pot containers and be forced to the ejection point by means of air pressure.

When the trigger is pressed only a little way, the air valve is opened and compressed air passes out of the nozzle. Further pressure suction feed draws back the needle, which runs through the length of the gun, and this permits the paint to escape. The rate of flow is governed by the pressure on the trigger; the harder the latter is squeezed, the greater will be the flow.

As soon as the paint begins to flow it is caught up by the stream of compressed air which emerges in the form of a hollow cone, within the confines of which the paint is atomised or broken up into minute particles. The shape which it assumes on leaving the nozzle depends on the design and adjustment of the latter. The type most widely used has three apertures:
one in the centre, through which the paint and air-stream issue, and one on either side. These last two are so situated that some of the air passes through them, hits the cone on opposite sides and flattens it to an elliptical shape. According to the adjustment of the nozzle, the stream of air with its atomised paint particles forms either a horizontal or vertical ellipse; or, if desired, another turn of the air cap will close the two holes on the sides, so that the stream of air is cone-shaped and the paint is deposited on the surface in a circle. This can be controlled so that it is possible to spray within fairly closely defined limits.

Choosing a Spray Plant.-In purchasing spray-painting equipment, there are various points which must be taken into consideration. The compressor is usually belt-driven, either by petrol engine or electric motor. The latter is quieter in action and free from fumes, but if the decorator’s business is in a rural district, where electric current may not be available in some parts, a petrol-engine drive is the obvious choice. Care should be taken to ensure that the compressor is capable of supplying a sufficient volume of air for the type of work envisaged, especially if it is intended to operate more than one gun at the same time.

Consideration must also be given to whether gravity-feed, suction-feed, or pressure-pot containers are to be used. A gravity-feed cup is excellent for many purposes but has obvious disadvantages for ceilings and similar overhead work. A suction-feed cup enables a change of color to be made - more quickly, since it is simply a matter of detaching the cup in use and substituting another. Pressure-pot containers hold far more material than either of the other two types, and consequently, on large-scale work, their use means the saving of a good deal of time which would otherwise be spent in having to refill. In addition, there is less weight for the operative to support-an important factor on a job which lasts for any considerable - length of time.

Preliminary Adjustments.-When first using the gun it will probably be necessary to carry out one or two adjustments. See that the nozzle is set tightly so that the air is not allowed to escape. If a paint container is fixed to the gun, test it to see that it is screwed securely home so that no leakage of paint can occur. In the lid supplied to the paint container will be found a small hole which must be kept clear so that air may enter as the quantity of paint in the container diminishes. Before proceeding to use the gun on any specified work, it is advisable to try out on a spare panel to see that it is functioning properly. Fill the paint container approximately three parts full of the material being sprayed and, as a precautionary measure, strain the paint into the container through muslin. Whilst no doubt the - material has been supplied quite free from foreign matter, at the same time

it is quite possible for dirt and dust to enter the can and cause a stoppage in the gun, and thus it is merely a matter of safety to perform the straining operation first.

Manipulating the Gun .-When commencing to use the spray gun the operator should consider his tool in the same category as a brush. if a heavy coat is to be supplied with an ordinary brush, a full brush is taken and the coating is applied freely. On the other hand, with a thin coat the brush is only charged with a small amount of the coating material and the application is very sparing. In the same way the application must be controlled when carried out by means of the spray. un this ease the paint is in the spray, but the rate of the flow can be regulated and this is accomplished by the manipulation of the trigger. With our experimental panel in front of us and the spray gun charged with the paint, the gun is brought to - within 6 to 8 in. of the surface and the trigger gently squeezed. This pulls the needle back and the air and the material are allowed to escape. The paint will be blown on to the panel in the form of a fine dust, which will settle on the surface without forming a homogeneous film, giving the appearance of a piece of fine sandpaper. If the trigger is squeezed a little further, more material will be ejected and the film will assume a moist appearance. As the trigger is squeezed further, the film will become wetter and wetter until the stage is reached when the amount of paint blown out will be too great and a run will begin to form, which will increase until the maximum pressure is applied to the trigger. All this is carried out with the gun stationary and will enable the operator to judge the feel of his trigger.

Once the operator has discovered the right pressure on the trigger of the gun, holding the gun stationary, he can move the gun across the surface to perform a stroke as applied by brush. The rate of movement will be found to work in conjunction with the pressure on the trigger of the gun. Thus, if the pressure is light and the paint ejected lightly, the rate of movement will be correspondingly slower, if a wet film is to be maintained. Conversely, if heavy pressure is applied to the trigger, ejecting a heavy coat of paint, the arm must be moved more quickly or otherwise the coat will run. A little practice will soon reveal the relation between pressure on the trigger and rate of movement across the surface, and after a time the operator will maintain this ratio automatically. In performing the stroke it is very important that the gun is kept at the same distance from the surface as accurately as possible from start to finish. The tendency during “~ this operation is to carry out the movement by means of the wrist, which is wrong. If the movement is carried out by means of the wrist, the nozzle of the gun will form an arc of a circle with the wrist acting at the centre. Consequently, where a flat surface is being sprayed, the distance between the nozzle of the gun and the surface will be greater at the two extremities of the arc than at the centre. This will result in uneven application of coats with a heavy band at the centre, fading away towards the edges. The whole movement should be carried out from the shoulder and the correct distance maintained from the movement of the body.

Now that the correct pressure on the trigger and the rate of movement over the surface have been gauged, the operator is in a position to proceed with general spray application. The gun is moved from left to right and vice versa, raising the arm each time so that each successive stroke overlaps the one immediately preceding. On paper this sounds a comparatively simple procedure, but in actual execution it is rather difficult. In the centre of the stroke there is a band of from 2 to 3 in. inside which a proper coat of paint is applied, provided that the gun is held at the correct distance. Outside this band the amount of paint gradually diminishes until, at the extreme edges, the deposit is extremely fine. In view of this, the operator has to perform his strokes so that they will overlap and the feather edges are eliminated. If this is not correctly carried out, the result will be extremely patchy, and once this occurs it is very difficult to obtain solidarity of colour; very often it means the application of extra coats. If ordinary care is exercised, however, it is quite possible practically to eliminate this trouble and with continued practice no difficulty should be experienced.

When spraying a surface from side to side, there is a tendency for the operator to keep his gun open at the end of the stroke and carry on with the next by making a circular motion. This is not correct, as this procedure causes a heavy deposit at the edges and can result in runs and curtains. This fault is particularly apparent if a small surface is being sprayed. The gun should be closed at the end of the stroke; this is accomplished by releasing the trigger for a moment and resuming the pressure when the position for the next stroke has been taken up.

Although the general rule adopted is to spray in a horizontal motion, certain surfaces are more easily sprayed in a vertical movement. Wherever practicable, it will be found an advantage to spray one coat horizontally and the next one vertically, as this method helps to relieve the surface of the “ribbed” appearance common where only horizontal spraying has been carried out. When spraying in a vertical direction, the gun has to be adjusted so that the maximum width of spray delivery is obtained, as naturally, if the gun is used to the same adjustment as for horizontal spraying, only a thin line of heavily applied paint would be obtained. The adjustment is obtained by turning the air cap through a quarter of a circle, which brings the two air holes at right angles to their previous position. This causes the fan of applied paint to be projected in a horizontal manner and not vertically as before. Thus, by manipulating the gun as described, but moving in a vertical manner, the maximum width of spray is obtained.

When the operator has practised the methods enumerated above and is able to apply his coats of paint evenly and without runs and curtains, and practically no “ribbiness,” he has mastered the elementary rules of spraying and is in a position to carry out the plain, straightforward work. The many varied and beautiful effects obtained by the spray gun are the result of manipulation, and the methods of obtaining these will be dealt with later.

Cleaning .-A great deal of the success of spray-gun application depends upon the maintenance of the gun in a perfectly clean state. In the course of spraying operations, all types of material will be applied, ranging from heavy-oil finishes on the one hand to cellulose lacquer on the other, and, if sufficient attention is not paid to keeping the gun in thorough working order, trouble is bound to result. One golden rule to be observed without fail is to clean the gun thoroughly at the end of each working day. It must be remembered that if cleaning is carried out while the paint is still wet in the gun and the pipe lines, the operation is comparatively easy. If, how-ever, the cleaning is not carried out until the following morning, the paint will be found caked hard, difficult to remove, and there is a possibility of damage to the apparatus. This applies particularly to pipe lines from the paint container to the gun-in fact it is practically impossible to clean these properly, unless it is done immediately, as, with the hard deposit caked on the interior of the pipe, the aperture becomes blocked and eventually the hose itself cracks. The few minutes thus spent at the end of the day will repay the operator immeasurably, and incidentally reduce costs brought about by stoppages and breakages.

If the spray gun has a container attached to it, all unused material left in the container should be emptied out and a small quantity of cleaning solution introduced. In the case of oil paints and varnishes, the cleaner may be white spirit, but where cellulose lacquers are employed, cellulose thinners must be used. White spirit must on no account be used, as this will cause a deposition of solid matter in the gun, which is very difficult to remove. With the cleaner in the container, take a piece of soft cloth and hold it in front of the air cap; when in position, press the trigger, which will of course release the air but prevent it from escaping through the nozzle of the gun. This will cause the air to be forced back into the cap or container and set up an aerating action which will effectively remove all traces of paint from inside the gun. If this action is carried out twice, a perfectly clean pistol will result.

In the case of a gun with a pipe line attached leading from a pressure or gravity-feed container, all surplus material in the pipe must be allowed to drain back into the container whilst the gun is swilled out with a suitable cleaner. Having cleaned the barrel of the gun satisfactorily, attention can be paid to the nozzle, where a certain amount of paint congregates. In order to carry out this cleaning operation satisfactorily, the air cap and the nozzle must be removed. This can be accomplished quite easily by the aid of the kit of tools supplied with the gun. In cleaning the air cap, particular attention must be paid to the two air outlets which are placed on either side of the outlet for the paint. These two holes are very fine in dimension and are liable to become choked quite easily, particularly if the paint is allowed to become quite set. However, if the cap is removed immediately after use and placed in a receptacle containing a suitable solvent for the paint used, the cleaning operation is accomplished quite easily. In the event of the air holes becoming clogged with solid material, the foreign matter may have to be forced out by other means. This is a delicate operation, as on no account must the air aperture be damaged. A pointed wooden skewer, with a piece of Cotton twisted round the point, should be used for the purpose and forced through the hole gently until the paint has been removed. A pin or similar stiff object should not be used, as it is liable to cause damage and distortion to the air hole, with consequent faulty spraying. The nozzle is cleaned in a similar manner and is more easily dealt with. Particular attention should, however, be paid to the seating in which the needle works. The seating must be kept free from hard material which is liable to cause damage to the needle. The success and good results of spraying are dependent to a large extent upon the needle fitting into the nozzle perfectly when the trigger is at rest. A number of faults associated with spraying may be traced to the fact that the needle does not set accurately into the hole in the nozzle. If it is desired to leave the gun for a period without cleaning, it is advisable to empty the container and leave a drop of the cleaning solution in the gun. This will prevent the paint from hardening in the latter, and thus prevent delay when it is required to resume operations. When the gun has been cleaned at the end of the day it is advisable to apply a few drops of Lubricating oil to all movable parts, as this will prevent undue wear on these parts and also help the gun to operate smoothly. Parts which should be oiled regularly are the trigger unions, the needle spring, and the seating in which the needle works. The oil must be of a light nature and applied sparingly as excess oil will find its way into the spraying part of the gun and thus spoil the material being sprayed, particularly if this is a nitro cellulose lacquer.

Where a separate paint container is in use which is connected to the spray gun by a delivery pipe, the pipe line will, of course, require cleaning This is quite an easy matter if carried out immediately after use. Plug one end of the pipe and introduce the cleaning solution until the pipe is filled,~~ when the feeding end is also plugged. The pipe may then be rolled up and left until required, when the cleaning solution may be drained out and the pipe line cleaned ready for use. Most pipe lines are made of rubber, the inside linings can be destroyed quite easily if care is not exercised in the choice of a cleaning solution. This solution should not contain any solvent for the rubber. One other part which requires attention is the cover on the container, which is fixed to the gun. An air hole is punctured in this which must be kept clear to allow the air to pass into the cup as the paint is sprayed from the gun.

Particular stress has been laid upon the necessity for cleanliness, as upon this depends the life of the spray gun and also the results obtained.

Protecting Surrounding Surfaces

An important factor in spray painting is the proper protection of surrounding surfaces and floors, and this may be carried out in various ways. Wherever possible, dust sheets should be placed, particularly over furniture or articles left in the room. The greatest amount of deposit will be found on horizontal surfaces, such as ledges and floors in the vicinity of the spray - operation, and this may be collected quite easily by the use of newspaper. - This is fastened flush with the bottom edge of the surface by means of adhesive tape. When operations are finished, the paper may be folded up and the dust carried out of the room in the paper. Care must be exercised in the choice of masking tape, and only qualities used which are specially recommended for the purpose. Glass may be protected by the use of masking solution, which is brushed on to the surface and then removed by a wet cloth. Another method of protection is effected by the use of a shield, which may consist of wood, metal, or composition board. It should be as light as possible, since it is held in position by the operator, and the edges must be dead true and straight so that a clean, accurate edge is obtained. The shield is held in the required position by the operator whilst the spray work is carried out with the other hand. The whole art of masking lies in carrying out full protection with the least possible labour and expense. Before commencing any spray work it is therefore advisable to carry out a survey of the work to be done from a masking point of view and plan out how this may be accomplished with the greatest economy.

Common Faults in Spray Painting

Unsatisfactory results in spray application may be due to a variety of causes, such as faulty technique on the part of the operative, or again, spraying at an incorrect pressure. In some instances they may arise from failure to keep the gun clean and in good condition; the following are those which most commonly occur:

Distorted Spray.-This may be due to a dirty or damaged air cap or from the tip not centering properly; to remedy, remove and clean the cap, paying special attention to the air ports. Should one of these be blocked with dried paint, the shape of the air stream and atomised paint will be irregular. If any parts are damaged, see that they are replaced.

Split Spray.-This may result from too much atomising pressure or from misalignment of tip and nozzle, or from an obstructed port.

Air Leakage from the Front of the Gun .-This is probably due to the air valve not seating properly and, provided no actual damage has been sustained by the gun, can be remedied by cleaning.

Fluid Leakage from Front of the Gun .-In this case the needle may not be seating correctly in the fluid tip, due to dirt or other impurities in the tip. Alternatively, the needle may be bent or of an incorrect size for the tip.

Fluttering Spray .-This is probably caused by an air leakage through the needle-packing gland; the trouble can be rectified by means of a new packing washer.

Pebbling .-This fault, in which the material is deposited on the surface too dry, as a kind of dust, is usually the result of spraying at too high an air pressure which causes the solvents in the finish to evaporate from the atomised particles before the latter reach the surface.

Orange-peeling.--.---This may be due to the material itself not possessing sufficient flow or to the use of unsuitable thinners. Again, it may be caused by the use of too low an air pressure in relation to the viscosity of the material, with the result that atomisation is imperfect.

Runs and Sags .-These occur when too much paint is projected on to the surface in any one area, due to incorrect handling of the gun, as when there is too much overlapping in succeeding strokes.

Various Decorative Effects

Once the function of the spray plant and a general knowledge of the best uses to which it may be placed are known, the user will be in a position to carry out the various effects which are possible in spray painting. The scope of the operator will be enlarged considerably and ideas may be carried out which are completely outside the scope of the brush. The remainder of this chapter will describe how the various designs are to be obtained, and with a little practice and ingenuity many pleasing and effective patterns are obtainable.

Masks and Templates .-Many interesting effects can be procured with the spray by the use of masks and templates, cut into squares, L-shapes, triangles, and other simple forms. They can be easily made from stout cardboard or any other suitable material which will lie flat against a wall or panel. They should be coated with shellac varnish so that oversprayed paint or distemper can be cleaned off without difficulty. As rigidity is essential, it is a good plan to glue on each mask a piece of plywood of the same shape but of slightly smaller dimensions. A small piece of wood screwed in the centre will provide a convenient handle.

By means of these masks an unlimited range of patterns and designs can be obtained. We can, for example, produce a decorative border with a piece of cardboard about 9 in. deep and i8 in. long, with a series of uniform serrations cut in one of the long sides. The wall is given a ground coat of the desired hue and the mask is held against it; the colour is sprayed on, working away from the edge, leaving a band outlining the serrations on one side and fading in intensity on the other. This, perhaps, is executed in a light colour. When the latter is dry, the mask is placed in position again, but dropped an inch or two or possibly moved slightly to the right or left, and a rather darker colour is applied. The result is a simple but attractive pattern which is continued by the same method along the length of the wall. Other effects will readily suggest themselves to the craftsman who experiments with the process.

Stencils .-The general conception of stencil work is a design made to a set pattern, ~s, for example, the well-known key pattern. Stencil work, however, can be developed a great deal further, particularly by the use of the spray gun, which has increased the range of effects enormously. The stencilling outfit is quite modest, a few set designs being all that is required, as from these, with a judicious use of colour values, a large range of effects is obtainable. The stencils should be cut from material which is sufficiently rigid to remain firm when the air is blown on to the surface. Any inclination to movement at the edges will result in smears and runs which will be difficult to efface.

The surface should be prepared in the usual manner, the final coat before stencilling being tinted to the desired shade to act as a ground coat for the colours to be stencilled.

To produce stencilled pictures, the use of two or three stencils may be necessary to build up the picture. A good example of this is in the production of a landscape where, for instance, three or our colours are required, such as green grass, brown for tree trunks, green for leaves, and blue for the sky. Each section will require its stencil and each stencil must be cut so that, when superimposed, the results form the complete picture without any gaps. This is accomplished by drawing the design and taking as many copies as the number of stencils required, which ensures that each copy is an exact replica of the others. Thus, if the stencils are cut accurately, when placed in correct position there should be no gaps, and when the picture is completed the appearance should be that of a freehand picture.

The first stencil is placed in position and the desired colours sprayed on to the surface. Correct manipulation of the gun is essential for artistic work, as by variation in the pressure on the trigger and the distance of the gun from the work, the light and shade and tone effects of the picture are produced. When spraying is completed, the stencil is removed and the paint allowed to dry out before the second stencil is placed in position, and so the work proceeds until the stencilling is completed. The picture as it stands may appear rather flat, so to counteract this, touch-up work may be carried out. Thus the introduction of a few clouds in the blue sky or a slight variation in patches in the green of the trees or the grass will relieve any sameness.

This description of stencilling by no means defines the scope of the work possible, and various schemes both in colour and design may be thought out which will strike out a new line in decorative art.

Shading .-This method of treatment is essentially one adapted to the use of the spray gun; shading with the brush requires skilled handling and even in the hands of the expert can hardly produce results comparable with those obtained by the use of the spray. It must not be taken for granted that no skill is required in spraying, as this is not correct ; the greatest difficulty that presents itself is in the elimination of a streaky appearance, due to the variation of density in the band of colour produced by the gun. There is no difficulty in shading small surfaces,but the trouble is experienced when large wall flanks have to be treated in this manner. There is only one method of overcoming this difficulty and that lies in practice, which enables the operator to gauge the pressure on his trigger and the correct method of application.

Shading enables the decorator to introduce a number of shades into his colour scheme, ranging from variations in tone of one colour to the blending of various colours, one into the other. The ground colour is applied over the whole surface and must be the most predominating in the case of multicolour schemes, or the most predominating shade where variations of one colour are being employed. This ground coat may be applied either by spray or brush, but as the shading is carried out by spray, the presence of the spray outfit will probably ensure its use for the purpose.

The shading effect is produced by the way in which the coats are applied. Thus, when a deep tone is to be graduated into a lighter one, the heavier tone is applied by opening the nozzle and holding the gun at a distance of about 6 in. ; as the tone is required to be lightened, so the nozzle is closed and the distance of the gun from the surface increased. This change in application must be gradual, otherwise the “streakiness” already mentioned is bound to show itself, and once this appears it is very difficult to remedy. However, proper use of the gun will prevent the possibility of this trouble occurring.

There is no doubt that shading of wall surfaces has a definite place in decorative schemes, and wallboards in particular lend themselves to this form of decoration. They have the advantage over plaster surfaces in the fact that their texture is always the same; this cannot be said for plaster work, in which the decorator has often to contend with variations in the rendering of the plaster due to the work of the plasterer. Particular attention must be paid, however, to the porous type of board, and the application of the paint carried out with care, otherwise the work is liable to turn out patchy. Such results are due to unevenness of application and are in no way due to the wallboard surface; they must not be confused with the results, sometimes obtained on plaster walls, which are due to the variations of porosity in the plaster.

Mottled Effect .-The mottled effect is a two- or more colour scheme in which the various colours have the appearance of spots dusted over the surface to form a decorative effect. The ground coat is sprayed over the whole surface in a normal wet coat, so that perfect coverage is obtained. While this coat is still wet the second coat is applied on top in a different manner. The gun is held well away from the surface so that instead of the lacquer or paint reaching the surface in a wet state to enable it to knit up and form a homogeneous film, the atomised particles are partially dry and the film is built up in a dry powdery form. The ground coat is sufficiently wet to allow the dry particles to knit up in the film, and thus we have a normal, smooth film with a mottled effect. Of course, to produce the mottle it is necessary for the ground coat and second coat to be of different shades. Thus schemes can be built up from varying shades of the one colour, as, for instance, light and dark green, or two entirely different colours may be used.

In the rough-surfaced mottle effect, the ground coat is allowed to dry hard so that the second coat is unable to sink into it, and this remains on the surface. Otherwise the procedure is as already described.

The type of mottle obtained is governed by the viscosity of the material, the air pressure, and in the manipulation of the gun-that is, the distance at which the gun is held from the surface. The farther the gun is away, the finer is the dust obtained, and similarly the viscosity and air pressure can be regulated to satisfy individual requirements.

Marble Effects .-Attractive marble effects can be obtained simply and effectively with the spray, used in conjunction with a bunch of hemp or tow, such as is employed by plumbers for wrapping pipe joints.

The hemp is dipped in a fairly strong solution of glue, wrung out, and then laid on a board and individual strands pulled apart, so as to form a kind of network which should be as thin and flat as possible. This is allowed to dry, when it serves as a kind of stencil.

The surface under treatment is sprayed with a ground coat of white, grey, green, black, or other suitable colour. When this is dry and hard, the hemp stencil is placed over the surface and the veining colour sprayed through it. The markings thus imparted resemble those of marble.

This type of finish can be carried out in distempers, flat-oil paints, or cellulose enamels, diluting the material liberally for the lighter veinings. After a time the hemp will become somewhat clogged but can be washed out by means of a suitable solvent, according to the type of finish which is being sprayed.

Spraying Plastic Paint

The brush application and texturing of plastic paint has already been considered in Chapter 6, but while the methods described are suitable for domestic work, they are, generally speaking, too costly for the treatment of large wall surfaces, such as are found in cinemas, theatres, dance halls, and similar buildings. By means of suitable spray equipment, however, it is possible to apply the plastic material to the surface in a variety of textures straight from the gun; if a ready-tinted plastic paint is employed, a one- coat finish is consequently obtainable.

The gun for this kind of work has a larger tip and nozzle orifice, in order to allow a greater amount of material to be ejected. A pressure-pot-or pressure-cup for small-scale work-must be used and a special type is available for the purpose; in this the paint is forced out of a pipe in the base of the container, while an agitating device is fitted to ensure the material being kept in proper suspension.

In the great majority of cases, water-thinned plastic paint is used and is supplied in powder form. In mixing, the powder should be added to the water. The consistency is a matter of experiment, and the spraying pressure, size of the nozzle, and air pressure must be taken into consideration. The air pressure will depend on the consistency of the paint, and the stouter the latter the greater will be the pressure required.

In some cases, a thin, smooth coating is wanted, to be subsequently textured by hand-combing or other means. For this, the air valve is fully opened to permit full atomisation, as when ordinary paint or cellulose lacquer is being applied. If, however, a texture straight from the gun is wanted, the valve is kept nearly closed, the degree to which it is open depending on the effects desired. If it is maintained nearly shut, the paint is ejected in a series of blobs of more or less uniform size, giving a kind of pebble-dash finish. By opening it slightly more, the size of the blobs is decreased and their shape altered. The technique is not difficult and an experienced spray hand will find little difficulty in mastering it after a little practice.

Flock Finishes

An excellent imitation of felt, plush, velvet, and other kinds of fabric can be obtained by spraying various kinds of flock on a surface previously coated with a suitable adhesive. The flock consists of fine particles of cotton, paper, rayon, silk, or other similar materials. Most firms of spray- equipment manufacturers market a special gun for the purpose; this has a large orifice and the ordinary type of air-cap for producing a flat spray, and is used in the same way as is the gun employed for paint, lacquer, and other liquid materials.

Almost any type of surface can be effectively finished by this process. It should be clean and free from grease and the adhesive sprayed on with the normal kind of gun. The best type of adhesive is probably an air- drying synthetic-resin enamel, which should match in colour, as closely as possible, that of the flock which is to be applied. If a clear varnish is used there is some risk of parts of the ground showing through the flock when the work has been completed. Cellulose-base vehicles can be employed but are not to be recommended, owing to the rapidity with which they dry.

As soon as the surface has been coated with adhesive, and while the latter is still wet, the flock is sprayed on. Cotton flock is made in either ground or cut form. Ground flock, as the name implies, is ground in mills, while cut flock is sheared off to a specific length and is thus excellent for simulating velvet or plush, which have a distinct pile.

Ample time should be given for the adhesive to dry, after which the surface is lightly brushed to remove any loose or surplus flock. This type of finish is economical and is extensively used for card tables, jewellery cases, and other industrial purposes. Its possibilities for the house decorator are more limited but it is worth mentioning as an instance of the variety of effects which can be obtained by means of the spray gun.

Shot-silk Effect.-The effect obtained in dress fabrics of colours changing according to the angle of light may be produced by decorators under suitable conditions. The surface must be granular in appearance; that is, very rough and uneven, a condition which is specially produced in some wallboards, or again, by means of plastic paint.

Here again, the effect is produced by the skill of the operator, who, after a little practice, can obtain very pleasing results. A ground coat of one colour is sprayed in the ordinary way and allowed to dry hard before the application of the second coat. This next coat is applied by the operator holding the gun at an angle to the surface with the nozzle pointing either upwards or obliquely to the plane of the surface. The effect of this is that the projected colour, again different from that of the ground coat, hits only one side of the projection of the surface, the amount of the material being deposited varying with the degree of projection or roughness. Thus, very prominent projections catch more colour than the lesser ones, and this variation adds considerably to the general effect.

The shot effect is obtained as the observer views the surface, the colour changing according to the way in which the spray gun has been held, and very beautiful and pleasing effects are obtainable. The employment of metallic powders is well worth while, and the use of a pastel shade as a ground coat followed by a dust coat of aluminium powder is very fine. There is no limit to the effect obtainable, and this type of decoration will repay a little investigation on the part of the decorator.

Splutter and Spatter Finish.-Although this finish has been in existence many years, it is only fairly recently that any great use has been made of it. Previously its employment was confined mainly to the decoration of panelling for interior work. In this form of decorative effect there is no set design, the lacquer being applied as a series of lines and blobs, the form or shape being governed by the manipulation of the gun. The strokes or blobs are applied almost as fine as a spider’s web, or the reverse, according to the viscosity of the material and the air pressure employed, and the designs at the disposal of the sprayer are many and varied. In addition, numerous colours may be employed and superimposed one over the other to complete any colour scheme that may be desired.

To obtain the splutter effect, a different principle from that ruling in ordinary spraying practice is followed. In the latter case the success of the operator depends upon the proper atomisation of the material being sprayed. Thus the viscosity of the paint must be adjusted to suit the air pressure, so that the material is split up into such a finely divided form that a uniform film is deposited upon the surface. In the system under discussion the reverse principle rules, and the material must be ejected in a stout form, an apt illustration being that of icing sugar being ejected from an icing tool as used for the decoration of cakes. To obtain this state, the material is used in a thick consistency and the air pressure dropped so that the stream oozes from the nozzle.

Crackle Finishes .-It is often true that from the mistakes encountered during the search after the ideal new ideas are launched. In the case of crackle lacquers we have an example of this put into effect, for this modern and ancient finish is a glorification of the old cracked-varnish effect. Readers have no doubt seen a varnish finish which is a network of cracks, the nature of these varying from those of hair fineness to the wide gaping crevices. This phenomenon is, of course, due to the varnish being faulty, and partly due to age. It was found that, if a cellulose enamel was under- bound or over-pigmented, as the enamel dried off.so the film contracted until a point was reached when the stress and strain became too great for the strength of the film, and consequently cracks appeared. A new type of finish was suggested at once, and thus we have the original of the now familiar crackle lacquer.

This finish can be brought under control so that the spray operator is able to control the nature of the finish, provided that the lacquer is formulated in a proper manner. He can produce at will either the fine, delicate, hair-like cracks, or the bold, deep fissure, and any variation of cracks ranging between these two extremes. The type of article being finished will, of course, determine the nature of the fissures, as these must harmonise with one another. Thus, for small surfaces, such as door panels, the large cracks would spoil the effect; a delicate crackle appearance would be aimed at. A large object can be treated with fissures of varying size, used to enhance any particular part of it.

The essential of a crackle finish is a two-colour effect in which the colour to appear in the fissures is utllised as a ground Coat. Thus, if we take as an example the common black-and-gold effect, the gold is sprayed as a ground coat. When the ground coat of gold is dry, the black finish is applied, and it is here that the operator is called upon to use his spraying skill. The nature of the fissures can be predetermined by the distance that the gun is held from the surface being sprayed and also the nature of the film applied-that is, whether the film reaches the surface wet or dry. Thus, if the delicate type of finish is required, the gun must be held well away from the surface so that the enamel reaches the surface in a semi-dry state. The result will be a multitude of very fine cracks, which can be very attractive on a suitable article. The heavy type of fissure is obtained in the reverse way, and if the enamel is applied heavily, or in other words, piled up, the cracks are exaggerated and the surface can be made to resemble a relief map in miniature.

As this crackle effect is obtained by making the enamel faulty by using excess of pigment, it is only natural that if reasonable wear is to be obtained a protective coat must be applied, and this is carried out in the form of a transparent lacquer. This finishing coat may be either glossy or mat, the latter effect being particularly pleasing.

The whole success of a crackle lacquer depends upon the manipulation of the gun and also on an intelligent and artistic combination of colours. For the delicate type of crackle, the colours should harmonise and preferably be of pastel shades. A good conception of this may be obtained by using silver as a ground coat and employing a delicate pastel blue as the crackle. This finish with a mat lacquer gives a particularly pleasing and soft effect. Where the fissures are large, the colour scheme must be bold so that the fissures may be rendered more prominent. Thus, if a ground coat of black is used, a white crackle lacquer superimposed, and the whole finished with a glossy transparent lacquer, the effect is striking. Incidentally, the crackle lacquer will give perfect cover in one coat owing to the large amount of pigment present.

In the opinion of the writer, the crackle finish has not been exploited enough, but has been rendered mediocre by the stereotyped finishes produced. With a little initiative and the employment of artistic sense, many pleasing effects are obtainable, with a distinct sales value.

Bronze and Verdigris Effect.-This type of finish is most becoming on metal fittings such as grilles, radiators, and metal casements. The lacquer and bronze powder are usually supplied separately and are mixed on the job as required. To obtain the verdigris and shadow-coloured effects a further coat of lacquer tinted with the shading matter is applied over the coat of bronze lacquer. Care must be taken in applying the shadowing colours, as if these are put on too heavily the effect is spoilt.

Oxidised-silver Effect.-In a similar manner to that for verdigris, oxidised-silver effects may be obtained. The ground coat is aluminium lacquer or paint, and when this is dry the oxidised effect is obtained by applying a steel-grey enamel in the form of a fine spray on the necessary spots.

The foregoing finishes are some of the more popular effects, but these can, of course, be supplemented by the ideas of the decorator and a few hours spent with a spray gun and a little initiative. Modern decoration demands new treatment and with the help of the spray the decorator is in a better position to cater for the requirements of his client.

The “Bromak” System of Mechanical Painting

This system, which combines some of the advantages of both spray and brush application, and also avoids some of the disadvantages of both methods, has considerable possibilities for the painter and especially for the large-scale contractor who specialises in certain types of work.

The apparatus consists of an air-compressor set, which may be either petrol or electrically driven, a pressure paint container, and the necessary tube, with which is connected a specially shaped brush head. The paint is forced under pressure through the centre of the brush, its delivery being controlled by a button on the head-piece. Various interchangeable types and sizes of brush, suitable for all classes of work, are available.

Perhaps the chief advantage of the “Bromak” pneumatic brush is that lead paints may be employed with it in conformity with Home Office Regulations, which forbid their application by means of a spray. Other points in its favour, as compared with the traditional mode of brush painting, are that the waste motion of dipping the brush into the paint kettle is entirely eliminated, thereby economising in time, labour, and material, and that the flow of paint is strictly regulated. The pressure paint container is constructed to take the actual tin in which the paint is delivered from the manufacturer, making changes of colour an easy matter. The plant is simple to operate, easy to clean, economical in use, and, by its means, paint can be applied approximately 40 per cent. more quickly than by the ordinary brush.

A further point worth bearing in mind concerns priming: it is generally agreed that, even when it is intended to apply the remaining coats by spray, the primer-especially when new woodwork is being painted-should be brushed on, and many specifications incorporate a clause to this effect. When this is stipulated, some painters make a practice of spraying the priming paint on to the surface and then brushing it out, so as to force it well in, but in such cases the use of the “Bromak” process would be quicker and more convenient. The “Bromak” system was primarily intended for the application of heavy protective paints on structural ironwork, bridge work, gas-holders, tanks, wagons, and similar surfaces, but its use has since been extended, and with the latest models practically any type of surface, from canvas to steel, broad or narrow, can be efficiently treated. It is significant that among its regular users are the four English railways and many of the greatest industrial concerns in the United Kingdom.