Home >>Professional Decorating>>Decorating Museum

Decorating Defects - their cause and cure

Discoloration of Paintwork

The discoloration of paintwork is a complex subject because this defect may arise from a variety of causes, some of them obvious enough, but others not easily recognisable by the decorator.
It will be helpful if we classify it first under three main headings, as follows:
( I) Discoloration due to agencies exterior to the paint film.
(2) Discoloration brought about by the properties of the paint itself.
(3) Discoloration originating from the surface on which the paint has been applied.
Under (I) can be included dirt deposition in all its several forms, mould growths, the action of light and heat, and contaminated air; under (2) chemical or physical changes; and under (3) " bleeding," rust, and alkaline action. These will be discussed in turn.
Dirt Deposition.-One of the most common forms of discoloration from exterior agencies is that due to dirt deposition, and ordinarily the origin of the trouble is not hard to identify. The nature of the discoloration, how­ever, varies according to climatic and atmospheric conditions, local circumstances and other variable factors; on occasion, too, the trouble manifests itself so soon after application that it is difficult to believe it has been brought about by dust or dirt. It is naturally more common in towns and industrial areas, where the atmosphere is more heavily charged with impurities, than in country districts.
Dirt is deposited on the surface of paintwork by the action of air currents. There is thus as a rule a marked difference between the extent of discoloration, due to dirt, of surfaces facing the prevailing winds and that of more sheltered areas, though the effect of rain must be taken into account; projecting portions of a building may tend to collect more grime, yet the paintwork on them is not necessarily more soiled, since it is also more exposed to rain which washes the dirt away, often causing it to accumulate on adjoining parts of the surface. A good example of this is to be found in the blackening which frequently takes place not so much on, as under and around window sills.
Inside a house, discoloration brought about by dirt deposition is com monly found above and around radiators and other sources of heat; paint­ work (or wallpaper) inside a hot airing cupboard often becomes more disco loured than other parts of the wall surface in the same room, although the doors of the cupboard are kept shut for most of the day; this is because the warmer atmosphere inside the cupboard creates what is almost, in effect, a vacuum, which continually draws in fresh supplies of air, containing microscopic particles of dust, from outside.
Nature of the Paint Film.-The decorator may consider, with some justi fication, that he is powerless to prevent paintwork being discoloured from this cause and it is perfectly true that there is very little he can do about it.
But it should be observed that the extent to which atmospheric impurities ~.are retained by painted surfaces is influenced in some degree by the nature of the film itself. A paint which dries slowly and remains tacky for any considerable length of time is bound to be more affected than one which hardens in a relatively short time. Similarly, one which provides a soft film is more likely to suffer from dirt deposition than is one which dries with a hard and glossy surface. From the point of view of liability to discoloration in service, this fact is worth bearing in mind when the choice of finishes is under consideration, particularly in towns and industrial districts.
A striking example of this came to the attention of the author of this work a few years ago. A number of notice boards, written in black on a white ground, were executed and placed at intervals on land adjoining a big railway shunting yard. Some of the boards were painted with a slow­drying oil paint, others with a quick-drying white synthetic-resin enamel. Within four or five months the former were so discoloured from dirt as to be difficult to read from even a short distance away, while the latter, though slightly soiled, were, to all intents and purposes, as legible as when they had just been written. Admittedly the circumstances were exceptional, but in a modified form the same kind of thing can happen in domestic painting.
Sulphurous Compounds.- The nature of dirt deposited on paintwork will vary according to conditions of exposure, and a chemical examination of the deposit will sometimes give a clue to the cause of the trouble. In large manufacturing towns, for instance, the air is often contaminated with sulphurous compounds which will react with white-lead paint by converting the lead carbonate into lead sulphide and causing the finish to assume a drab or brownish appearance.
This may be more or less uniform over the surface-though the more protected parts of the building are less likely to be affected-or the trouble may show itself in the form of streaks or spots. It may be brought about by particles of soot, containing sulphur, settling on the paintwork, and is especially apt to occur after heavy fog. The flatter the surface of the paint, the worse the discoloration will probably be, because in a flat paint the pigment particles are more exposed to the atmosphere, whereas in a gloss paint they _are more protected by the medium. The stability of the paint film is not seriously affected and the discoloration can be removed by the application of a solution of hydrogen peroxide without much difficulty, but the effect is, of course, unsightly.
In all cases where blackening of paintwork has occurred, the possibility of it being due to dirt should first be investigated and the surface, or a portion of it, washed with a solution of sugar soap or other cleansing medium, preferably applied warm. This may sound an obvious precaution to take but the fact remains that it is sometimes omitted, especially when the dis­coloration takes place soon after application and is thought by the painter to be the result of chemical action or to some defect in the paint itself.
Pattern Staining.-Before we leave the subject of dirt deposition, there is one other aspect which demands special attention.
Most decorators are familiar with the curious darkening, almost like an X-ray photograph, which takes place on ceilings, and seems to indicate the position of the beams and laths-above. This phenomenon, technically known as " pattern staining," was the subject of Bulletin No. 10, issued by the Building Research Station in 1931; from the information contained in the latter, it is clear that pattern staining usually occurs in buildings in which the air above the plaster is for long periods at a different temperature from that in the room below, as, for example, in the 10ft above the ceiling of a church. In such a case, the plaster under the laths is warmer than on the rest of the ceiling, and, in consequence, there are corresponding differences in the air pressure acting on smoke particles floating in the atmosphere; the result is that more particles are driven towards the colder parts of the ceiling, so that" ghosts" of the laths appear as white bands on a darker ground.
The remedy is to equalise the temperatures of the different parts of the ceiling, either by the use of specially thick plaster or of special paper of " honeycomb" construction, or by other means of a similar nature.
This defect is far more common than might be supposed and a great deal of time, material, and trouble has been expended in recoating ceilings affected by it, in an effort to eliminate the discoloration. Within a short time, however, it reappearsed its prevention and cure are, in fact, beyond the capacity of the painter and outside the scope of his work.
The Action of Light and Heat.- The discoloration or fading of paint­work from the action of light is a very big subject and can be dealt with only very briefly in this work.
Attempts are often made to classify pigments according to their degree of permanence to light, and though this can be done to a certain extent, too much faith must not be placed in it,. since their behaviour in the paint film depends on so many varying conditions. For example, the medium with which the pigment is mixed may exert considerable influence on the retention of the original colour, as may also the atmospheric conditions to which the paint is exposed. The same paint, applied in similar conditions to surfaces of a similar nature but in different parts of the country-at the sea­side, in an industrial district, or in a rural area, for example-may show remarkable differences in fading after a year's exposure. Another point to bear in mind is that a pigment which, in a pure state, stands the light perfectly may fade if used in admixture with another pigment (which may, by itself, be equally light-fast). or when reduced with a white basic pigment and used as a tint. It may be said, however, that the more protection the pigment portion of a paint receives from the direct rays of light, the less fugitive it is likely to be; hence, paintwork protected by a final coat of varnish will probably retain its original colour, as modified by the colour of the varnish, better than will a flat or nearly flat coating.
The action of light on the medium must also be taken into consideration. Light has a bleaching effect on linseed oil and other drying oils used in the preparation of paint, and on occasion this may contribute to any slight apparent change in the colour of the finish.
So far as the effects of heat are concerned, the temperatures in Great Britain are generally relatively low, so that, in respect to discoloration, the action of heat on exterior work can be more or less disregarded. Hot surfaces, or those exposed to heat, within buildings present special problems though these are generally more concerned with the durability of the finish than with any possible discoloration. It may be said, however, that ordinary white or light-coloured paints will tend to darken or yellow in such circumstances, due mainly to the action of heat on the driers. When surfaces of this kind have to be painted, manufacturers who can provide special heat-resisting finishes should be consulted and, if possible, they should be informed of the maximum temperature which the paint will have to with­stand in service.
Effect of Alkaline and Acid Fumes.-In interiors in which certain manufacturing or commercial processes are carried on, the air may be charged with alkaline fumes which may attack the paintwork unless careful consideration has been given to the choice of pigments and media. Ignorance of this fact often leads to trouble when painting is done in, for instance, ladies' hair-dressing establishments where, in permanent waving, fumes of an ammoniacal nature are usually present. Certain pigments such as Prussian blue, chrome yellow, and Brunswick green (which consists of a mixture of Prussian blue and chrome yellow) are readily affected by alkalis and discolour badly.
Alternatively, similar colour changes may take place if paints containing certain other pigments are exposed to acidic fumes. Ultramarine, for example, is quickly decomposed under such conditions.