The importance of the primer, or first coat, can hardly be exaggerated, and the durability of the entire finish depends very largely upon its ingredients and manner of application. Some painters, who should know better, look upon it almost as a formality and regard almost any kind c paint as good enough for the purpose, their argument being presumably that because it will be covered up its composition is immaterial. In the same way, they seem to imagine that no skill is necessary in applying a priming coat, and consequently give this kind of work to labourers or apprentices.
Needless to say, this point of view is entirely wrong and is almost certain to cause trouble. If the primer fails, everything fails, and those with experience will agree that not only must this first coat be of good quality, but it must be of the correct type for the work in hand; it must, moreover, be properly applied if subsequent coats are to give proper protection for any reasonable length of time.
It is only fair to add that the painter is by no means the only offender in this respect. Many manufacturers of mass-produced doors, window frames, etc., appear to be equally oblivious to the necessity for applying reliable priming coats to their goods, and on housing estates in course of construction it is no uncommon sight to see stacks of doors, frames, and other wooden articles exposed to the elements and coated only with some rubbishy wash which offers no protection whatever against the weather. It must be emphasised that the application of unsuitable primer to bare wood, not only fails to preserve it, but involves expensive cleaning off if a reasonably permanent finish is desired.
The function of a priming coat will differ, to some extent, according to the nature of the surface to which it is applied. (On ferrous metals, for example, one valuable purpose may be to inhibit corrosion; or again, on new cement, to prevent alkaline salts from attacking the finish.) Broadly speaking, however, a primer must adhere well to the surface and itself provide a ground to which subsequent coats will bond firmly. It must also possess sufficient elasticity to contract and expand according to the movements of the ground on which it is used; if it is lacking in this property, cracking will soon take place, and the protective value of the whole paint coating will thereby be impaired.
It should be noted, however, that obliteration of the surface is not a primary objective of a priming paint and no special effort to do so should be made. A primer which completely obscures the ground to which it has been applied is nearly always over-pigmented and consequently too brittle for its purpose. Nor, it may be added, should a priming paint be expected, by itself, to possess great weather-resisting properties, though it should have sufficient to protect woodwork from becoming too damp, or iron and steel from rusting, should these materials be exposed to the elements – as not infrequently happens on building sites – for weeks on end. But while it is important that it should be capable of withstanding a certain amount of exposure, it does not possess the degree of impermeability afforded by a good finishing coat and consequently should not be allowed to stand too long in the open air before the undercoat and finish are applied on top of it.
Paint adheres to a surface in one of two ways. On an absorbent ground, as on woodwork or plaster, it does so by a limited penetration of the pores, the bond being substantially strengthened if the surface has a certain amount of ‘ tooth ‘ or texture which, in many instances, can be imparted to it by rubbing down with an abrasive. On a non-absorbent ground, as on glass or polished metal, paint clings by what is known as ‘ molecular adhesion,’ an essential factor of this being that the paint must be of an inherently tenacious nature.
It follows from this that the composition of the primer will depend largely on the degree of suction of a surface. On those which are highly absorbent, a paint containing a high proportion of oil should be used, in order that the porosity of the ground may be satisfied and the voids filled; if this is not done, there will be a tendency for the oil to sink into the surface, separating from the pigment portion, leaving the latter insufficiently bound. On grounds which have little or no suction, the paint vehicle should be of a more viscous nature, such as a varnish or bodied oil. Volatile thinners (e.g. turpentine or white spirit), besides enabling the material to be properly spread, contribute to its penetrative powers.
It is essential that the pigment should be very finely ground; if the particles are too coarse or flocculent, a filtering effect will take place on a porous surface and too much of the binding liquid will strain away through the pores, leaving too short a layer of pigment on the outer surface. On the other hand, if the pigment is finely ground, it will penetrate along with the liquid just within the surface pores and help to seal them against too deep a penetration of the vehicle. In this way a good and durable foundation will be obtained.
Priming paints should be fairly thin in consistency and should be well brushed out, care being taken that the brush is thoroughly worked into quirks and the angles of mouldings so that these are neither bridged over nor choked up. As a general rule, spray application of the primer is not recommended and even when the remaining coats are to be applied by spray, it is fairly common practice to specify that the priming paint should be brushed on. Although much will depend on the composition of the paint, the angle at which the gun is held, and the distance maintained between nozzle and surface, it is probably true that, in most cases, better adhesion is obtained by brush application.
Types of Priming Paints
For exterior surfaces, the best known and, in the opinion of most competent authorities, the most satisfactory type of priming paint is made on a white-lead basis (with or without a limited amount of extender), raw linseed oil, turpentine or white spirit, and driers. For certain kinds of surface – notably most species of woodwork – a small proportion of red lead is incorporated; this acts as a drier, but its chief purpose is to harden the film which, without it, tends to be rather soft, though this is not necessarily a disadvantage. The addition of the red lead gives the mixture a decided pink colour and hence it is commonly referred to as ‘ pink primer.’ It should be noted, however, that this description does not necessarily imply that the paint is made up wholly or mainly on a genuine white- and red-lead base, for unfortunately many of the ready-mixed, lower-grade primers sold under this name are made with cheap and inferior pigments, tinted to resemble the lead type.
The excellence of white-lead primers is due to many reasons. They adhere well to the surface because, in addition to lending itself to fine grinding, white lead has a remarkable affinity to linseed oil, so that the pigment particles are well ‘ wetted ‘ and bond firmly together. White-lead priming paints have good elasticity and a high degree of moisture resistance. Compared with other forms of primers, however, one of their most valuable properties is the way they can be worked under the brush; this enables them to be forced well into the surface and to be spread out in a thin and even film.
Zinc oxide and lithopone are not regarded as so satisfactory as a basis for exterior priming paints since, under conditions of exposure, the hard film which they give is apt to crack and scale. For priming interior surfaces, however, they are extremely useful ingredients. They are capable of extremely fine grinding, have good adhesion, and provide an excellent foundation for the hard-gloss type of finish.
The use of aluminium powder, mixed with a suitable medium (generally a varnish or treated oil) for priming paints has gained in favour of recent years, though such paints do not as yet rival the traditional white-lead primer in popularity. It should be pointed out that all aluminium paints are not necessarily suitable for priming pur- poses and that, as in the case of most other materials, they vary considerably in quality. The powder should be finely ground and the choice of medium needs special attention.
The chief value of aluminium primer lies in its high degree of resistance to moisture. This property substantially reinforces the protection given by the entire paint coating on exterior surfaces, minimises warping of woodwork, and the fact that movement of the latter is thus reduced imposes correspondingly less strain on the film. The relative degree of impermeability of an aluminium primer tends, however, to make it rather more prone to blistering than are other paints, when used on surfaces which are not entirely dry. Other types of primer will usually allow a limited amount of moisture to pass slowly through them without substantially affecting their adhesion. Aluminium primer, on the other hand, will effectively seal up the moisture, but if the paint film is subjected to the rays of the sun, the pressure thus engendered may lift the primer from the surface in the form of blisters.
The addition of aluminium powder to priming paints is a practice which is finding increasing favour, and experiments which have been made over a good number of years show that, in nearly every case, the paints are thereby improved. It gives greater hiding power and, owing to the nature of the aluminium flakes, which overlap like fish scales, strengthens the paint film. The waterproofing properties of the paint are undoubtedly improved. Pigments such as lithopone certainly gain by its incorporation, while tests show that even white lead may benefit from its inclusion. Being inert, aluminium has no harmful effects on other pigments. From 10 to 20 per cent, of the powder, which should be microscopically flaky, is usually added for this purpose.