Woodwork which is dry and well seasoned, reasonably free from knots, and carefully prepared by the carpenter or joiner, provides an admirable ground for paint; its porosity will vary according to the species and condition of the surface, but as a rule the latter will have sufficient suction and texture to permit good mechanical adhesion of the primer. Conversely, if the wood is not properly matured or is damp, or again, is too resinous, it may give considerable trouble to the painter since it may offer only poor adhesion to the paint film, which may soon develop blisters and other defects.
Painting on woodwork is mainly confined to the soft woods; until the 1914-18 war, joinery in this country was largely made from deal, but since that period large quantities of Columbian pine (otherwise known as Oregon pine or, in the U.S.A., as Douglas fir) have been imported for the purpose; this wood does not, on the whole, take paint so well as does deal, and special care is necessary in coating it.
The Structure of Wood
Before paint can be successfully applied to any surface, it is desirable that the nature and structure of the surface should be well understood and this is especially so in the case of woods since they exercise a considerable influence on the subsequent behaviour of the paint film. If we examine a cross-section of a tree-trunk, we shall see that it consists of a central pith (or medulla), surrounded by bands, alternately light and dark in colour, of cellular matter; the light bands represent spring-time growth and the dark, late summer and autumn growth. Spring wood is less dense and more absorbent than the other and holds moisture more readily; in consequence, moisture drawn to the surface tends to find its way out via the spring wood. As a result, paintwork over these areas is more likely to develop blisters than is the rest of the surface.
We shall also see, in our cross-section, a number of streaks radiating outwards from the medulla; these are known as medullary rays and they convey nourishment from the central pith, through the heartwood to the outer layers, or sapwood. The use of the latter is generally avoided in good structural timber, but unfortunately much of it is employed in lower-grade woodwork; being soft, subject to shrinkage, and prone to hold moisture, it makes a poor foundation for paint.
Knots form a mass of woody fibre which runs at an angle to the main stock. They are characterised by a number of ducts which permit the passage of resin and consequently give trouble to the painter, though some knots are far worse than others in this respect since they exude matter more or less continuously and will cause blistering of the paint film unless they are effectively dealt with. The treatment of knots will be considered later on in this section.
Wood unprotected by paint is capable of absorbing a great deal of moisture in a relatively short time, particularly if the end-grain is exposed. The drying-out process is apt to be deceptive, particularly if the timber is at all thick or has a fairly dense grain, and it often happens that woodwork which seems from an examination of the surface to be quite dry and fit for painting is, in reality, far from being in this condition. The moisture will evaporate from the outer cells quite quickly, but the transference of the moisture from the inner cells to the outer, and thence to the surface, takes far longer.
Wood which is unseasoned or only partially seasoned will undergo sub-stantial dimensional changes, because its fibres swell or shrink according to whether they take in or give up moisture. When it has been properly seasoned, the changes are less violent, though they still take place to some extent. Even painting cannot entirely prevent them, though it can keep them under control. It is important, therefore, that joinery work should be painted as soon as possible.
Effect of Wood Structure on Paint
It is common knowledge that paint will last longer and wear better on certain species of woods than on others, even when the wood in each case is properly seasoned and not unduly resinous. Probably for a year or two, this fact will not become apparent, but when the paint film begins to deteriorate and lose its elasticity, the nature and structure of the wood will play almost as important a part in determining the rate of disintegration as will the composition of the paint itself.
There is a great deal of variation in many species of wood in both density and ring growth, and consequently a corresponding variation in painting characteristics. One American authority has given it as his opinion that density and ring growth are a far more reliable guide to the way in which paint will behave than is the species of wood, but, as he points out, wood is bought and sold by species and not by its structural properties.
It should be added that many other factors in the treatment of wood may affect the behaviour of the paint applied to it. For example, assuming that boards are in contact with damp surfaces, such as brickwork, from which they can absorb moisture, blistering of the paint coating may occur, but it is less likely to do so if the boards are of the flat-sawn variety than if they are quarter-sawn.
Before the priming coat is applied to woodwork, knots and resinous places must be treated or blistering and disfigurement of the finish will almost certainly occur.
The treatment almost invariably specified is to apply two coats of knotting, but in really bad cases this usually proves inadequate since it fails to hold back the resinous exudation for more than a short time. It is far better, in such instances, to take out the knots bodily with a brace and bit and replace them with sound wood. This, of course, takes far longer than applying knotting, is much more expensive, and requires a good deal of skill if a neat job is to be made, but where time and costs permit, it is certainly the safest procedure. Alternatively, bad knots may be gouged or burnt out well below the surface of the surrounding wood and the cavity filled with hard stopper.
It is sound practice to play the flame of a blow lamp lightly on any suspected part of the wood to determine whether any resin exudes when subjected to heat. When burning out a knot, a good idea is to use a thin sheet of asbestos with a hole bored through it as a shield for the woodwork around the knot.
In most cases, however, the painter will have to rely on the application of knotting. The best is made from pure shellac and alcohol (usually methylated spirit); the special advantage of shellac is that it is insoluble in turpentine and oil and therefore will not be dissolved by the liquid portion of the primer applied on top of it. ‘Patent ‘ knotting is made from certain resins, such as manila copal, with or without a proportion of shellac added; it is not so reliable as shellac. Cheap grades of knotting often contain rosin, which is soluble in turpentine. Since the object of knotting is to protect paint from the action of the rosin in the wood, it is obviously foolish to use a knotting which contains this very substance.
Disadvantages of Knotting
Provided a good-quality knotting is used and the knots are not bad examples of their kind, the application of two thin coats usually serves its purpose well enough in isolating resinous areas from contact with the paint; it is a relatively cheap, simple, and speedy operation, for shellac varnish (which is the same thing as shellac knotting) dries very quickly. Yet it is not an entirely satisfactory process, and instances of paintwork cracking or blistering over areas where knotting has been applied are not uncommon. No doubt these are due, in some cases, to the use of an inferior-quality material, but even when the best is employed trouble may follow and it does not need much imagination to discover why.
When knotting has been applied to certain areas of woodwork, it is clear that the porosity of these areas is less than that of the untreated parts. If the priming paint which follows contains a fairly high proportion of oil, as it probably will do, this will be absorbed by the bare portions of the wood but not by the areas which have been knotted. Over these areas, therefore, the film of paint will retain more oil and take longer to dry, and if the next coat of paint is applied before they have hardened properly, they will remain permanently softer and consequently will provide places where blistering may occur; if a hard-drying finish is used over these areas, there is some risk of the film cracking.
Moisture Resistance of the Primer
A priming paint for woodwork should have good moisture-resisting properties. No paint gives a permanently impervious film; at the best, minute quantities of water pass through it, in and out of the wood, bringing about slight dimensional changes in the fibres nearest the weather-side. These changes impose a certain amount of strain on the adhesion of the paint, and the more they can be prevented the longer the life of the paint is likely to be. Many paints provide a more or less impermeable coating when first applied, but one which becomes progressively more porous fairly rapidly: a good primer should remain an effective barrier to damp for a considerable period.
Nowhere is moisture-resistance in a wood primer more important than in the treatment of joinery work, such as door or window frames, which come into direct contact with brickwork. In new buildings, this brickwork is saturated with water and it is highly desirable that all woodwork which abuts upon it should be well protected with paint, to prevent it from absorbing the moisture. In cheap building, this back-priming is sometimes omitted or, which is almost as bad, a poor-grade paint is employed for the purpose. The result is that the moisture, periodically renewed by rain, finds its way into the woodwork and attacks the finishing coats from underneath. Unless back-priming is conscientiously carried out with good-quality paint when the building is erected, failure of the finish will almost certainly take place within a short time and will continue to recur when the finish is renewed.
Any exposed end-grain of timber needs special attention because of its ability to absorb water. End-grain is invariably extremely porous and if a single coat of primer is applied there will be a strong tendency for the medium to be drawn deep into the wood, leaving the pigment underbound on the surface. Whenever possible, therefore, double-priming should be resorted to.
Choice of Primers
Broadly speaking, the best all-round primer for exterior woodwork is probably a genuine white-lead paint, with or without the addition of a small proportion of red lead. The amount of oil should be adjusted according to the porosity of the wood: it is a fairly common belief amongst painters that the adhesion of paint to woodwork depends largely, if not entirely, on the depth to which it penetrates; actually this is not entirely correct. Paint certainly does penetrate wood and in doing so performs a valuable function by filling the voids; it imbeds itself in the surface pores, but beyond this the depth to which it sinks has little or no effect on adhesion.
There is much to be said for the use of aluminium primers on woodwork, although a great deal depends on the choice of a suitable medium. The latter is nearly always a varnish or treated oil and consequently the action of this type of paint is rather different from that of the traditional white-lead primer in that there is less penetration of the surface. The resistance by a good-quality aluminium paint to the passage of moisture has already been referred to and, in addition, it has the necessary degree of flexibility and adhesion.
Priming Columbian (or Oregon) Pine
Because of its remarkable brushing properties, and also because they are more accustomed to it, most painters prefer a lead primer and certainly, on good-quality deal, which presents few variations in surface structure and is of moderate and more or less uniform porosity, it gives excellent results. On resinous woods, such as Columbian pine, in which layers of hard grain alternate with softer and more absorbent areas, it is less successful and aluminium primer is probably better. This wood, it may be added, often presents something of a problem to the painter: it is given to resinous exudations and sometimes shows beads of gummy matter on the surface. These must be removed before priming by scraping and scrubbing with turpentine or other suitable solvent, and the oil content of the primer kept on the low side. In bad cases, it may be necessary to apply a sealing coat of shellac to pre-vent the resin from forcing its way through the paint film. Most of the leading manufacturers supply special primers for this species of wood, and it is generally more satisfactory to use one of these than to rely on ordinary paint mixtures. A further difficulty is that the grain ridges are very pronounced and unless filler coats are applied will show through three, four, or even more coats of paint.
The advantage of adding a proportion of aluminium powder to white lead and other forms of priming paints has already been mentioned. The white pigment should be mixed first with the oil and drier and the aluminium flakes should then be added, thoroughly well mixed, and the whole then thinned to brushing consistency with turpentine or white spirit.
Hardwoods as a general rule are stained and varnished or polished, though occasionally oak sills, pitch-pine posts, and similar articles are required to be painted. None is likely to give much trouble to the painter, with the possible exception of teak which, on account of its hardness and durability, is sometimes used for high-class joinery work. It may cause difficulty because it contains a natural oil which continually rises to the surface and stains and attacks a paint or enamel finish unless measures are taken to prevent this happening.
The treatment recommended for this wood is, first, to remove as much of the oil as possible from the surface by wiping the latter with a cloth moistened with turpentine; in bad cases, a stronger solvent, such as acetone, is advisable. After wiping clean and allowing plenty of time for any solvent residue to evaporate, a priming coat of high gum content gold size, thinned with an approximately equal amount of turpentine, should be applied; it should be put on liberally and worked well into the grain. This should be left overnight to dry and then a further coat of gold size, but without thinner, should be applied. When this is dry, it should be flatted and is then ready for the undercoat. As an alternative to the gold size, shellac varnish can be used; the first coat should be a wash coat, consisting of about 2 lb. shellac to a gallon of methylated spirit, while the second should contain only about half as much solvent. After the first coat, paste filler should be applied.
Water Paint as a Primer for Woodwork
Strictly speaking, woodwork which is at all damp should never be painted, but there are occasions when this has to be carried out in spite of the probability of the film blistering. To minimise this risk, it is sometimes recommended that instead of an oil paint, a water paint primer should be used, since the latter provides a porous coating which, owing to its nature, can absorb a certain amount of moisture from the wood without much ill effect.
It is quite true that this treatment is not infrequently successful in preventing blistering, but nevertheless it cannot be considered sound practice. Water paint is a useful material as a wall coating, but it makes a poor foundation for coats of oil paint, since it lacks the properties of good adhesion to wood, flexibility, and resistance to moisture which, as has already been stated, are essential to a good primer for woodwork. It may provide some sort of a safeguard against blistering, but it cannot be expected to compare in durability with a good oil paint, and a paint system based on a coating of this kind is far more liable to premature failure by cracking and flaking than is one based on more orthodox methods; once it begins to break down, moreover, disintegration will be comparatively rapid.
Because a water paint primer is often effective in preventing blistering of the finish when the wood is damp or unseasoned, it should not be assumed it will be equally successful in doing so when there is a danger of the defect occurring from other causes. When, for instance, wood which is unduly resinous has to be painted, it is useless to apply a water paint primer in the hope of checking the formation of blisters, since the coating will have little adhesion and will lack the mechanical strength to hold back the resin.
Using Wallboards In Interior Decorating
Wallboard is an invaluable material in present-day building practice, and the number of different types which have been introduced during the past few years is evidence of their popularity. Their structural possibilities need not be emphasised, for, generally speaking, they lie beyond the province of the decorator, but since the latter is often called upon to paint or paper ceilings, walls, and other surfaces for which they have been used, their treatment is of some importance to him.
Wallboards are made from a variety of substances, which include wood fibres, sugar-cane, cork, plaster, and a host of others. They differ considerably in colour, porosity, and texture, and thus the same kind of treatment cannot be applied to all of them, for the characteristics and special properties of each must be studied. In most cases, the manufacturers issue definite instructions as to preliminary treatment, and recommend certain methods and materials, and these should always be followed.
The difficulties which usually arise in the painting of wallboards are often beyond the control of the decorator, since they are due, in the first place, to incorrect methods of fixing the boards in position, and of making the joints or concealing them when they are made. The most common practice is to butt-joint the boards, arranging wooden battens or other supports at suitable spaces for this purpose; the edges are adjusted as near to each other as possible and nailed to the support, the counter-sinking of the nails or screws being filled with ordinary paste or plaster stopper. Finally, the holes and edge-joints are hidden with a strong continuous strip of paper, which is intended to conceal them and also as a safeguard against the joints gaping if the boards show any tendency to shrink under the action of dry heat.
The defects which frequently occur in the painting of wallboards can usually be traced to differences in porosity between the board itself, the strips used for concealing the joints, and the material employed for filling the counter-sunk holes. The paper or linen strips are often far less absorptive than the rest of the surface, with the result that, whatever type of finish is applied, they dry out with a different colour and gloss. The discrepancy, which is occasionally very conspicuous, can be minimised by selecting a type of jointing paper on which the paint will dry out with as nearly the same effect as on the board, or, alternatively, by decorating the strips separately with a flatter type of paint than that used for the remainder of the work.
Nails or screws should be sunk as deeply as possible, and the cavities thus formed filled with plaster or stopper, or else with a paste made by scraping a waste piece of the actual board to a rough powder and mixing his with a solution of glue size. Press small portions into the holes, squeezing out any excess size; allow this stopper to dry hard and then sandpaper off level.
Certain types of wallboard are too thin to lend themselves to this treatment, and the most satisfactory mode of concealing the joints is probably by means of mouldings. Whenever these are employed, the boards should be painted right across the joints, before the mouldings or battens are affixed. If this is done, it will prevent the appearance of an unsightly bare strip of wood on either side of the batten, in the not improbable event of the boards shrinking. There will be the further advantage that no laborious ‘ cutting-in ‘ will be necessary at the edges of the battens, which can be painted separately and affixed when dry.
Discoloration of the finish can occasionally be traced to nail heads, and although it more often occurs when the wallboard is coated with water paint or distemper, it is by no means unknown when flat oil paints are employed. Most wallboards are capable of absorbing a certain amount of moisture, and, if they are affixed to a damp wall, the nail heads may be attacked, and, where those of the flat-headed variety are used, there may be discoloration from iron nails in the form of rust, or even from brass nails as greenish or brownish patches.
Unfortunately, priming or varnishing the nails with ordinary air-drying materials appears to be only a temporary safeguard against this defect; nails specially prepared to resist the effects of moisture should be used whenever possible.
Great care should be exercised in the handling or storing of boards before erection, or in hammering in nails, that no dents or surface injuries are inflicted. Damage of this kind is apt to be overlooked until the boards are decorated, when it is too late to rectify it.
Woodwork or other surfaces which have originally been polished, and are now to be painted, require special preliminary treatment, even if it is a considerable time since the polish was last applied and there is no superficial evidence of its survival. The slippery feel of the surface, and its tendency to polish up under light friction, are proof that there is still some left, but even if these tests reveal nothing, it is wise to assume that there are traces in existence.
The precautions against it are simple: heat some turpentine by placing the bottle or jar which contains it in a vessel of hot water and allowing the latter to simmer. When the turpentine is hot, dip a clean cloth in it and go thoroughly over the surface under treatment. Then repeat the process with a solution of soda, or sugar soap, and water, and while the work is still wet, scrub well with waterproof abrasive, finally rinsing off with clean water. A medium-grade waterproof sandpaper, which will leave the surface slightly scored, is probably the best abrasive to use.