Up to the war of 1914-18, it was customary to apply at least four coats of paint on new exterior surfaces in good-class work and even more might be given where special protection was required. Generally speaking, the results were good but the four-coat process was possible only because, at the time, the cost of both labour and materials was relatively low. After the war, when wages had risen steeply, materials were more expensive and speed in completing contracts had become of paramount importance, the practice of giving only three coats became more common and may be said to have been accepted as standard.
Most authorities in this country take the view that three coats on new outside work must be regarded as the minimum; each coat has a definite function to perform. The first, or priming coat, must satisfy the porosity of the surface, adhere well to it, and itself provide a ‘ key ‘ for subsequent coats. The main purpose of the undercoat is to give opacity but it, too, must adhere well to the primer and afford a ground of uniform suction for the finishing coat. The latter provides the required colour and a more or less impervious film as final protection against the elements; since its resistance to the weather depends to a considerable extent on a high proportion of oil or resin in its formulation, it is almost invariably a gloss paint or enamel.
Where, instead of a gloss paint or enamel finish on outside work, a paint and varnish finish is preferred, the minimum, for new surfaces, is generally held to be four coats – primer, colour coat, and either two coats of varnish or a second colour coat (made up to dry with a semi-gloss finish), and one coat of varnish.
In cheap work, attempts are often made to reduce the number of coats to two but the results are not usually very satisfactory because they fail to take into consideration the fact that each coat has its own particular part to play. As a rule, the undercoat is omitted and, to counteract the loss of opacity which this involves, either the primer or the finish, or both, are given a higher proportion of pigment. In many instances this is unsuccessful since the primer becomes too thick and stodgy to adhere properly, and the finishing coat too porous to have more than a limited resistance to the weather.
The Two-coat System
It should, however, be added that in America in recent years a two-coat system for new exterior work has found a certain amount of favour; the principles on which it is based are rather different from those of the three-coat process; in the latter, the primer is usually of a rather thin and oily consistency, in order to penetrate the surface pores, whereas in the two-coat system, a bodied oil or varnish-like medium is used for the primer, which contains a rather higher proportion of pigment. This system has been employed with reasonably good results in this country for interior work, but up to the present its possibilities for outside work have not been very seriously considered. It is significant that in The Painting of Buildings, a booklet prepared for the Ministry of Works by a committee convened by the Paint Research Association, and published in 1944, three coats are held to be the minimum number for new exterior surfaces.
For repainting outside work where the existing finish has weathered but is still in sound condition, two coats are generally held to be essential – an undercoat and a finishing coat. A single coat is not as a rule satisfactory, even when no change of colour is wanted, because on every surface certain parts are more exposed than others and consequently portions of the paintwork have deteriorated more and, as a result, are more absorptive. If only one coat of paint is applied over a ground of this kind, the new finish is liable to be patchy and more glossy in some areas than in others, because the porosity of the surface is not uniform all over.
In outside painting, the main objective is protection from the elements, but in inside painting the question of protection is naturally not of so great importance, and on new work more than three coats are seldom applied, except in high-grade enamelling, of which, however, comparatively little is done nowadays.
In ordinary interior painting, the chief function of the primer is to satisfy the suction of the surface and provide a good ground for the undercoat which is sufficiently pigmented to afford good obliteration. The purpose of the finishing coat is purely decorative – to provide the colour and degree of gloss required. For repaint work inside buildings, two coats are usually regarded as necessary, especially when a change of colour is wanted.
Principles of Coat Sequence
In any successful painting job, one of the essentials is that each coat should not only be made of good-quality materials properly compounded, but should also be of such a nature that it will combine with and agree with the others, so that the whole will form a strong and homogeneous film able to withstand the strains and stresses to which it will be subjected in service.
The flexibility of a coat of paint depends in the main on the proportion of vehicle to pigment which it contains. Within certain limits, the higher the percentage of oil, the more elastic the film will be and, conversely, the greater the proportion of pigment, the more brittle. The functions of each coat in an ordinary three-coat job have been briefly outlined above. The primer will be fairly thin and oily so that it can penetrate the surface pores and thereby gain adhesion; the undercoat will contain less oil and more pigment because its role is to obliterate, and the finishing coat will be mixed with a high percentage of oil which will confer upon it gloss and flexibility.
In the old days when four- or five-coat jobs were common, the golden rule was that an oily coat should be followed by one less rich in oil, and vice versa, and this principle still holds good; there is. however, an alternative view which holds that each coat should be progressively more elastic, which means, in effect, that in each, the proportion of oil – either by itself or in the form of an oil varnish – should be slightly increased. Although these two rules seem to some extent to conflict, both are perfectly sound in practice.
Those who advocate the system of progressive elasticity usually do so on the ground that to sandwich a flat coat between two oily ones is to run the risk of inducing cracking because the flat coat, being by nature less flexible than the other two, is unable to contract and expand to the same extent. But, in point of fact, the danger of this is so small as to be practically non-existent. What happens in such a case is that a kind of adjustment takes place automatically after application, the flat coat absorbing a certain amount of oil from the others, thereby decreasing the difference in elasticity.
In cases where premature cracking of the finishing coat occurs, the trouble can usually be traced to failure to observe the rules of coat sequence. Probably the most common mistake of this kind is to apply a relatively short final coating over a more flexible undercoat; the brittleness of the former prevents it from conforming to the movements of the latter and the surface tension thus created is too great for the finishing coat. In grained work, it frequently happens that this defect takes place after a few months’ exposure, because the varnish used, though it may be of excellent quality, is not sufficiently elastic for the graining colour on top of which it has been applied. It may be added that the principle which brings about this failure is sometimes deliberately exploited to produce striking decorative effects, as with the so-called ‘ crackle ‘ finishes. In these, an extremely short, quick-drying finish is used over an elastic ground coat of a harmonious or contrasting colour; in drying, the finishing coat splits uniformly all over the surface, revealing parts of the underlying coating through the fissures.
It is important that the principles of coat sequence be kept in mind, for there are occasions when they may easily be overlooked by the inexperienced. When, for example, in order to obtain extra protection and build up a good, solid finish, four coats are given on new exterior work, care should be taken that the second of the undercoats used over the primer contains more oil than the first. Yet although these principles demand either that successive coats be alternately oily and flat, or progressively more elastic, there are one or two exceptions to this rule. Certain types of flat wall finishes, for instance, can safely be applied in two coats without any adjustment being made to the oil content of the second coat; again, with many present-day synthetic-resin finishes, it is perfectly sound practice to apply a second coat of gloss enamel without even flatting down the first.