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Home >>Professional Decorating>>Decorating Museum>>Defects in Varnish
Decorating Defects - their cause and cure
Defects in Varnish
As previously remarked in this work, varnish is an extremely sensitive
material and is very easily affected by adverse conditions, both in storage
and until such time as the film has dried and hardened. It is subject to a considerable number of defects, some of which may also affect paints and other finishes, but some, also, which are for the most part confined to varnish.
The following notes enumerate as concisely as possible the principal faults which manifest themselves in the application of varnish, and explain
the causes of these defects, and where possible suggest some means of prevention and remedy.
Going Seedy, Sandy, or Bitty.-Definition.
-These terms are applied when the varnished surface has the appearance of having been sprinkled more or less with sand or fine grits.Causes.-Varnishing in too cold an atmosphere.
Using a varnish which has been stored in a cold place and has consequently become chilled.
Sudden change of temperature during the varnishing operation, in the painting-room, or in the place where the work is done.
Draining too closely a can which has been some time in stock.
Air-dust falling upon the surface while the work is in progress.
Broken skin in the varnish.
The presence of nibs, skin, etc., in the varnishing brush.
To Prevent.-All varnishes, however well matured, show a slight deposit with age, and therefore the" bottoms" of a can should always be reserved for odd jobs or mixing. Some varnishes, when stored for any length of time, become covered with a skin which, if broken, will produce a specky appearance under the brush.
Always strain finishing varnish through muslin before use. See that the brush is perfectly clean. This should be tested by varnishing a piece of glass; held up to the light, the presence of skin, nibs, or other impurities will at once be evident.To Remedy.-Rub off the nibs gently and lightly with a pumice stone which has been properly' prepared by filing off the sharp corners and edges.
This will prevent the nibs being torn up and so breaking into the surface. Then with the felting pad rub down the work carefully and apply another coat of varnish.
Sweating-up.-Definition
." Sweating-up" is the technical term applied to the oily gloss which appears on varnished surfaces after pumiceing.Causes.-Allowing too long an interval to elapse between pumicing the undercoat and applying the finishing coat of varnish.
To Prevent and Remedy.-All oil varnishes and black japan, from the nature of their composition, have a tendency to sweat, that is, to become slightly greasy upon the surface. This is especially the case in the highest class of coachwork and motorwork, where to ensure perfect flow and finish the painting-shops have to be kept at a high temperature Sweating probably gives more trouble in practical varnishing than any other cause, for the reason that this" sweat, if not removed, has the most deleterious effect upon the coat or coats that follow.
Each succeeding coat should therefore be applied immediately after the rubbing down. If from any cause this cannot be done, and the work has been allowed to stand, it must be gone over again very lightly with pumice powder and water, immediately before the next coat is applied. This precaution is of the first importance, as it will effectually remove the sweat and materially conduce to first-class results.
Sagging, Festooning, Enamelling, or Shrivelling.-Definition.
These are allied terms and denote a running down, or falling into curtains or festoons as the varnish dries, and a more or less uneven and in some cases a shrivelled appearance of the finished work.Causes.-Unevenness or bad preparation of the undercoats. Overbinding the colour undercoats. Varnishing in too hot or close a room.
Applying the varnish unevenly, thus leaving on certain parts of the surface a surplus which must necessarily wrinkle or " curtain" in drying.
Not working the varnish out sufficiently with the brush.
Applying too heavy a coat.
Varnishing before the undercoats are sufficiently hard.
Exposure of the varnish to cold draughts.
Thinning out the varnish with turpentine or oil.
To Prevent.-Shrivelling or enamelling through overbinding may be prevented by thorougly sponging the coat of colour with turpentine and water, mixed in proportion of about a gill of turpentine to a pail of water. Never tamper with varnish, but use it as received from the manufacturers.
To Remedy.-Rub down very carefully with pumice stone until the surface is quite level, then apply one or more coats of varnish, as may be necessary .
Pinholing, Pocking, and Blotching.-Definition.
-Pinholing: The varnish surface is disfigured by a number of tiny holes.Pocking and Blotching: The appearance is similar, but the holes are deeper and larger, and in the case of blotching irregular and overlapping. Causes.-The atmosphere of the varnishing-room being charged with moisture.
Mixing two varnishes of different qualities.
Effect of ammonia fumes on work m progress.
Varnishing over paint or varnish which has not hardened sufficiently, or which has sweated.
Using varnish too soon after receipt, before it has had time to recover from the effects of transit, such as agitation, chill, etc. Applying on a warm surface varnish which has been stored in a cold place; and vice versa.
An uneven temperature or want of ventilation in the varnishing-room. Sudden lowering of the temperature in summer causing condensation on the work in progress.
The presence of oil or turpentine on the varnish brush.
Varnish from the bottom of a can which has been some time in stock.
To Prevent.-See that the atmosphere of the varnishing-room is dry and warm. Should the temperature suddenly fall in warm weather, heat up the room at once, suspending the work until all moisture has evaporated. Varnish showing a tendency to pinhole arising from various causesparticularly changes of temperature-may frequently be entirely cured by " rouncing "; that is, taking two varnish cups and pouring the contents from one to the other ten or twelve times.
To Remedy.-In cases of pinholing and slight pocking or pitting it will usually suffice to rub down to a level surface with pumice and apply a fresh coat of varnish, but where there is deep pitting or blotching It may be necessary to rub down to the colour coats and do the work of varnishing over again.
Deadening or Sinking-in.-Definition.
-These and similar terms, such as " going sleepy," etc., are used to describe a defect which implies that the recently applied varnish has lost its gloss and become dulled, sleepy, or lacking in lustre.Causes.-Not thoroughly priming and filling. Absorption of the varnish by the undercoats, especially in one-coat work; that is, when a coat of hard-drying varnish has not been put on over the black japan, or colour work, before the finishing coat is applied.
Inferior quality and absorbent nature of the colours used in the undercoats. Reds particularly, as well as ultramarine blues and greens, frequently cause this trouble.
Hasty work, which often means varnishing over paint or varnish undercoats which have not been allowed time enough to harden.
Using varnish taken from a can which has just been opened and from which the natural gases have not had time to evaporate.
Using varnish which has been thinned with turpentine or oil.
To Prevent.-Use first-class filling, and be sure the colours are of the best quality and properly bound.
Always apply at least one coat of hard-drying varnish over black japan or colour, and rub down immediately before the finishing coat is put on.
To Remedy.-Rub down the unsatisfactory work lightly with pumice powder and apply another coat of finishing varnish.
Cbipping, Flaking, Scaling, or Peeling.-Definition.
-These terms all indicate the partial separation, in more or less marked degree, of one coat of varnish from another, or from a coat of colour; or of any other part of the work.Causes.-Inferior filling.
Ammonia fumes.
Omission of, or carelessness in, rubbing down between coats. Want of proper order in the application of the several undercoats. Painting or varnishing upon unseasoned wood.
Not allowing enough time between coats, so that one or more have not
sufficiently hardened.Applying the varnish over colours of inferior quality, or which have been eIther overbound or not sufficiently bound.
On old work, varnishing on a surface from which all traces of soap or alkali used in washing down have not been entirely rinsed off.
To Prevent.-See that each coat is thoroughly dry and hard, and that it is applied in proper drying sequence, and take care each varnish coat is carefully rubbed down before the next is applied.
To Remedy.-In slight cases of chipping or flaking rub down the affected coats with glasspaper or pumice, and build up the work again. With scaling or peeling from the wood itself it is generally necessary to burn off and start afresh.
Cracking.-Definition.
-The cracking or breaking up of the varnished surface into more or less diminutive sections.Causes.-Employment of inferior filling.
Faulty preparation of the undercoats. If these are too oily in character they will work under the varnish and cause cracking.
Want of method in building up the work; that is, undercoats not in proper sequence.
Undercoats not given sufficient time to harden properly, so that a greasy mess is sealed in by the finishing coat, with the result that, upon expansion of the undercoats by heat or contraction by cold, the finished surface is torn into cracks and fissures.
The action of alkalis or of the fumes of ammonia on the varnish.
Finishing with a varnish which is not sufficiently elastic.
Build up the work always from a "presto" priming through coats of graduated dry to hard undercoating varnish, To Prevent.Finishing with an elastic coat of varnish.
To Remedy.- The only effective remedy is to pumice down until a perfectly unbroken and level surface is reached and then build up the work again.
Blooming.-Definition.
-A whitish or foggy appearance of the varnished surface.Cause.-Smoke or dampness in the varnishing-room. Foggy or damp weather.
Want of ventilation, causing condensation in the varnishing-room. Varnishing under cold, damp, or ill-ventilated conditions, such as often occur in churches, chapels, public halls, etc.
To Prevent.
-As far as possible avoid varnishing in unsuitable weather or unfavourable conditions. In churchwork particularly, secure reasonable ventilation, and, if possible, some warmth.
To Remedy.-In coach- and motorwork, when the undercoating varnish only is affected, the trouble is not serious, and will disappear under pumice. In case of the finishing coat, if the bloom is of recent origin, it can generally be removed by thoroughly sponging with water. Even should the bloom reassert itself, one or two further spongings will remove it entirely.
In decorative work where the bloom upon the finished varnish is pronounced, and repeated sponging with water is impracticable, the defect may be remedied by lightly applying with a rag a mixture of linseed oil, methylated spirit, and vinegar, ill equal parts. Great care must be taken afterwards to clean off the surface thoroughly by sponging with cold water.
In severe cases, where the bloom has been neglected and allowed to become fixed, it may be found necessary to rub down the work with pumice flour and apply another coat of varnish.
Mud-spotting.-Definition.
-This term is used to describe marks upon the varnished surface which persistently appear after the vehicle concerned has been sponged and washed in the usual way.Causes.- This trouble occurs almost exclusively with coachwork, and is due as a rule to the motor car or vehicle having been taken out before the varnish has had time to become thoroughly hard and set; or in the case of older work, to the mud having been allowed to dry and remain for some time on the surface before washing. It is specially prevalent in towns, where the mud usually contains a certain amount of ammonia, and in country districts where the soil is chalky. To Remedy.- The stains may sometimes be removed by washing the work repeatedly with cold water, but in most cases it is necessary to go over the spots with linseed oil, a little of which should be applied with the finger tip, gently rubbed into the defective places and allowed to remain for tWenty-four hours before washing off. In severe cases this process may have to be repeated two or three times before the trouble is eradicated. This method is generally effective, but in extreme cases, where the spots have been allowed to stand and eat into the surface, the only remedy is to rub down the part affected and give another coat of finishing varnish.
A surface which, from mud-splashing or other cause, has become discoloured, and the gloss in places impaired, may often be effectually revived by the application of oil, spirit, and vinegar, as described under the head of " Blooming".
Blistering.-Definition.
-The swelling up in the form of blisters of certain parts of the varnished surface after it has dried and hardened.
Causes.-Unseasoned or resinous wood.
Want of care in properly priming and filling; or in preparing metal surfaces.
Employment of inferior filling.
Too quick work, so that insufficient time is allowed for each coat to harden properly .
The presence of sweat on the work when the paint or varnish is applied.
Exposure of recently varnished work to extreme heat, as when a car or carriage newly finished is allowed to stand exposed under a fierce sun.
Washing newly varnished work in sunshine.
Scale upon metal. To Prevent.
-Preparatory to painting, ironwork should be thoroughly searched for scales by going over it with turpentine to which a small quantity of linseed oil has been added. Then the surface should be hammered lightly all over to remove scale and finally sandpapered. Aluminium should always be most carefully sandpapered and all under-mouldings roughened with a file.
See that each application from priming to finishing coat is hard, and that it is fully rubbed down before the next is put on.
To Remedy.-Metalwoik: A blister should be cut round with a sharp knife; a piece of wood should then be placed over it and the surface of the wood struck with a hammer. This will jar the blister off. To build up the work again, the underlying rust must first be removed and a coat of sharp priming put on; follow with two or three coats of hard filler, so as to bring the surface above the surrounding level. Stone down carefully and finish as usual.
Woodwork: Very slight blistering may be treated in a similar way, by cutting out and scraping down to the wood, and filling up again to the varnished surface; but where this trouble is very pronounced there is really no satisfactory way of counteracting it, short of cutting down with pumice to the level of the lowest blister and building up the work again.
Running or Cissing.-Definition.
- The varnish, instead of flowing out upon the undercoat, runs from the brush, leaving irregular patches uncovered, which defect is not remedied by repeated brushings.
Causes.-Grease or sweat upon the undercoat.
Chill.
Ammonia fumes.
In old work, grease or dirt upon the surface.
To Prevent.-In old work thoroughly clean the surface, before varnishing, with water in which soda has been dissolved (never with soap), and well wash off with cold water.
In varnishwork which has been allowed to stand, immediately before the application of another coat go over the work very lightly with pumice powder and water to remove any sweat.
With grained and similar oil-colour work which has been allowed to stand and upon which the grease has had time to assert itself, go over the work with two coats of diluted animal size (a couple of handfuls melted in a bucket of hot water, but applied cold), which will give a surface with a sufficient key or grip to hold up the varnish. Another method, where animal size is not available, is to rub over the work with powdered whiting, which kills the grease and affords a sufficient key; but this process is not recommended, as although the whiting is finely levigated, its minute particles are apt to give the finished surface a seedy appearance.
To Remedy.-In the case of old work, as soon as the trouble manifests itself clean off while wet the varnish already applied, with American turpentine; see that the surface is thoroughly clean and then varnish. With grained and other oil-colour work, clean off the varnish while wet, size as described, and varnish when quite dry.
If the varnish which has run or cissed has been allowed to set, then the work must be pumiced down to the colour and revarnished.
Lining, Roping, and Silking.-Definition.
-Parallel lines more or less pronounced, which show upon the surface of the varnish when it dries.As they appear more or less closely together they take the name of" Lining," " Roping," or" Silking." A similar defect is known as " brush marks." Causes.- Varnish that has, on account of age or from being stored in too cold a place, become round or stout.
Employment of fattened varnish from a badly corked can.
Chill or cold draught at the time of application.
" Laying-off" or working varnish too long under the brush.
To Prevent.-In the case of a varnish which has stoutened with age, the addition of an extremely small quantity of old linseed oil (preferably slaked with lime) will ensure a perfect flow. In the case of varnish stored in a cold place, slightly warming so as to bring the bulk to a temperature of about 65 to 70 degrees will prove effective.
It is an excellent plan to bring the varnish can into the varnish-room at least twenty-four hours before it is opened; by this precaution the temperature of the varnish and the room assimilate. If this cannot conveniently be done, the immersion of the can in a pail of warm water is recommended. Brush marks are produced by working the varnish too long under the brush, and usually in cold surroundings.
To Remedy.-Rub down the work carefully until the surface is level and then apply another coat of finishing varnish in a suitable temperature.