How to Paint Previously Painted Surfaces

So far we have dealt only with the priming of new or unpainted surfaces, but by far the greater part of the work of the average decorator will consist in renovating those which have already been painted one or more times. If all buildings were properly looked after and skilled men, using only high-grade materials, were called in at regular intervals, repainting would be a comparatively simple and straightforward business, for old paintwork which has not been permitted to weather or disintegrate to excess provides an excellent foundation for new paint. In practice, however, relatively few structures are cared for in this way and, as the result of this neglect, the work of redecoration calls for the exercise of even more skill and experience than does the treatment of new surfaces.

There is a strong tendency among the public to attach too much importance to price and not enough to quality, especially in the matter of painting. It is fairly common to find property owners calling for four or five estimates from different concerns and giving the contract to the firm which submits the lowest tender, irrespective of how the work will be executed. True, the number of coats to be applied and the brands of paints to be used may be stipulated, but no specification can ensure that the all-important preparatory work is done as it should be. Another unfortunate consequence may be that in the course of years the work may be entrusted on each occasion to a different firm which has no knowledge of the methods or materials used by its predecessors and thus must be handicapped in dealing with any faults which may have arisen; it is manifestly a great advantage to be familiar with the history of a job and to know exactly what has been done to the surface in the past.

The first thing to be done before an estimate for repainting can be given is to examine the condition of the existing paintwork and to decide whether after proper preparation it is still sound enough to be used as a ground for fresh paint, or whether it must be removed and the surface re-primed and treated as if it were new work. There will be some cases where it is clear that removal is the only practicable course, as when the film is badly cracked or blistered or when partial flaking has taken place and there are indications that the adhesion of the whole coating is none too good. Removal will, of course, greatly increase the cost of the job but it will prove far cheaper in the end than painting over a coating which has no real hold on the surface and which is liable in a short time to break away from the latter, bringing the new paint with it.

Burning Off

The removal of old paintwork from woodwork is in the majority of cases accomplished by burning off which, despite the claims sometimes made by manufacturers of paint removers and solvents, is still probably the quickest and most economical method. To handle a blow lamp effectively needs a certain amount of experience and deserves more care than is commonly given it. A skilled craftsman should be able to use the lamp and stripping knife in such a way as to leave the surface free from scorch marks or burns or from islands of paint. Scorching is to be avoided not only because, if serious, it leaves depressions in the wood which may have to be stopped, but also because it may cause a local variation in the suction of the surface. Skimming over the paint so as to remove only the upper portion of the film is equally to be avoided, since the adhesion of the paint that is left is doubtful and, even if it is good, extra sandpapering will be needed.

The most common faults of the inexperienced in burning off are digging the blade of the knife into the wood and scarring the latter, and allowing the flame of the lamp to get too far ahead of the knife so that on those areas on which the flame has been playing, the paint cools off and hardens again before the knife gets to it. Digging the knife into the wood is the result, in most instances, of holding the tool at the wrong angle; it should be kept as flat to the surface as possible. The speed at which the knife moves should control that of the lamp though care should, of course, be taken that the flame is not directed at any one place for too long.

Burning off should be executed with the grain of the wood, not across it, and should begin from the bottom, with the knife held just below the flame as much as possible; it should be done methodically and not in haphazard fashion, straying without system from one part of the surface to another. One great advantage of the lamp is that it enables matter to be brought to the surface and removed from knots and resinous places; the more of this matter which can be eliminated in this way, the less the risk of blistering of the paint film.

Burning off is impracticable where adjoining surfaces, such as glass, may be damaged by heat, and it is inadvisable to employ this method of removal over plaster or cement; these materials are liable to contract if the lamp is played upon them so that, in the case of plaster, the skimming coat may crack and detach itself from the undercoat. On paintwork over iron and steel and metals in general, burning off is largely ineffective, since the paint tends to chill off too rapidly to yield easily to the knife.

Paint Removers

The stripping of old paint on most surfaces other than woodwork will consequently involve the use of paint removers. These are of two main kinds, the caustic type and the solvent type.

The use of caustic paint removers over woodwork should be avoided although, because they soften paintwork quick and effectively, and are in-expensive, they are widely employed for the purpose. But they leave behind in the pores of the wood alkaline residues which even repeated rinsing often fails to shift and which are liable to attack fresh paint when the latter is applied to the stripped surface. In theory, the addition to the rinsing water of an acid, such as vinegar, will counteract this, but in practice it will usually be found either that the acid is too weak to have this effect or that it is so strong that traces of it are retained by the woodwork with disastrous results. The fact that caustic removers, containing as they do a large proportion of water, raise the grain of the wood and thu necessitate additional sanding, is another objection to their use.

For the same reason it is undesirable to employ caustic removers for stripping paint from any surface which has a considerable degree of suction. Over impervious grounds, such as iron, there is less risk of subsequent trouble, though thorough rinsing must always take place after the paint coating has been removed.

An efficient and undoubtedly the safest method of stripping old paint is by means of paint removers of the solvent type, based upon blends of such ingredients as acetone, butyl alcohol, or tetralin; these dissolve the film without raising the grain of the wood or leaving behind them any acid or alkali to attack the new paint. Most of them, however, contain a proportion of wax which serves the double purpose of preventing the mixture from running on vertical or sloping surfaces, and of retarding the evaporation of the highly volatile solvent; traces of this wax are apt to be left on the surface after the old paint has been removed, and since they may inter- fere with the drying of the new finish, it is necessary to wipe over the surface with a rag moistened in turpentine or petrol before any repainting is begun.

The most common fault made by painters in using these spirituous paint removers is to follow on with the stripping knife too soon after the solvent has been applied and before it has time to do its work and, in this connection, it is as well to remember that some brands remain wet and working longer than others. Where old, thick paint, consisting of many coats, has to be stripped, it is usually necessary to remove it in layers, which may take a considerable time.

Solvent-type removers have their limitations and, on large surfaces, their use is apt to be expensive, but the fact that they leave no damaging after-effects is a strong point in their favour.

Removing Other Types of Finish

Since painting may have to be carried out at times on surfaces finished in varnish, distemper, or in some other material than paint, and the existing finish may have to be stripped, it will be convenient here to outline the methods commonly adopted for this purpose.

Varnish

Old varnished surfaces will, in general, be treated in the same way as old paint films. Where, however, the existing varnish coating has badly disintegrated it is sometimes possible to remove it by means of a strong solution of sugar soap which, for application to vertical or sloping surfaces, can be bodied up by the addition of flour. If this treatment is adopted, it must be followed by thorough rinsing with clean water, in view of the alkaline nature of sugar soap, since any residue left on the surface may attack the new finish.

Spirit varnishes, such as shellac, can be stripped by means of methylated spirit.

Distemper and Water Paint

Old coatings of size-bound distemper yield readily to water and present little or no trouble to the decorator. Washable water paint, however, which has to be stripped, often causes difficulty. It is a common experience to find in coatings of this kind which are flaking, and which must be stripped before it is safe to redecorate, patches which adhere tightly to the plaster and are extremely hard to remove.

Unquestionably, by far the best, quickest, and cheapest form of stripping old films of this description is by means of the steam-generating machines primarily designed for the removal of old wallpapers. Steam speedily loosens the distemper, which is scraped clean from the surface.

Where this apparatus is not available, other methods of removal must be followed, depending on the nature of the binder of the distemper. This varies greatly according to the brand, and since the painter is usually ignor- ant of what it consists he is often at a loss to know the best way to strip the old coating. In some instances, solutions of hot water and soda or sugar soap will be effective, in which case the addition of a little vinegar to the rinsing water is advisable, in order to counteract the effects of the alkali. In others, it may prove helpful to add a small proportion of ammonia, or in others again to use a weak acid solution (e.g. dilute vinegar). In practice, it is largely a matter of experiment to determine the best treatment to follow.

It is well known that some materials, such as glue size or flour paste, contract strongly in drying and, in doing so, exert a pull on any coating on which they have been superimposed. It has more than once been suggested that one way of dealing with old distemper which has to be removed, but which adheres tenaciously to the plaster in places, would be to apply a coat of strong size or paste, the idea being that the force of the shrinkage would be sufficient to cause the distemper underneath to be dislodged. In point of fact, this treatment is seldom attempted, but it might be worth while trying in obstinate cases.

Wax Polish

Wax polish applied to paint, varnish, stained or plain woodwork must be removed if any of these surfaces are to be painted or varnished, or the drying of the new finish will be prevented. Since wax polish consists of wax dissolved in turpentine or white spirit, the latter is an obvious solvent. It is most effective if used warm, the heating being accomplished by placing the container in a bucket or vessel of water over a slow fire or low gas jet.

Where large areas have to be treated, this method is apt to prove expensive. A cheaper alternative is to use a solution of sugar soap in hot water, with the addition of a little paraffin (about pint to 2 gal. of the solution). Constant stirring is necessary to prevent the paraffin from lying on top of the water, so that too much of it is taken up at a time by the brush or sponge. Thorough rinsing should follow the removal of the wax.

French Polish: This can be removed by swabbing the surface with methylated spirit.

Rubbing Down

In many cases an examination will disclose that it is quite unnecessary to remove the old paint, particularly in interior work, since such defects as have occurred only affect the surface of the film and do not extend throughout its entire thickness. All that is essential, in such cases, is to rub down and clean the surface by means of glasspaper (commonly known as sandpaper), pumice stone, or some other form of abrasive. This will serve to remove all perished material from the outer surface of the paint film, and at the same time afford proper adhesion for the new paint.

Under the provisions of the Lead Paint (Protection Against Poisoning) Act of 1926, no painted surface may be rubbed down or scraped by a dry process. The object of this regulation is to diminish the risk of lead poisoning; when white-lead paint is rubbed down dry, a considerable amount of dust is created, and there is obviously a danger of the painter inhaling this as he works. Since the Act of 1926 has been in force, the number of cases of lead poisoning among house painters has appreciably decreased.

Whatever form of abrasive is used, therefore, it must be employed in conjunction with a liquid. Ordinary sandpaper speedily disintegrates in the presence of moisture, and has been, in consequence, largely superseded by waterproof sandpaper, though in cases where the latter is not available there is no objection to the use of the ordinary type, provided always that some form of fluid is employed. When a considerable area has to be rubbed down, the sandpaper may, with advantage, be folded over a block of wood or cork which will make it far less tiring to hold. Better still, a sandpaper holder, properly shaped to afford a comfortable gripmay be used.

Many decorators – particularly those of the old school – are averse to wet rubbing down, because they believe it to be more expensive and messy than the older process.

There is much to be said in favour of turpentine, or turpentine substitute, with a little raw oil added, as a rubbing fluid, particularly when a single priming coat is concerned. In such a case, if water is used, there is more than a possibility that it will enter the wood and raise the grain, so that the purpose of rubbing down will be largely defeated. The mixture of oil and turpentine, on the other hand, will help to ‘ feed ‘ any spot on the woodwork which may be laid bare during the abrasive process.

Natural pumice stone, once extensively used in the trade, has been largely superseded either by sandpaper or by artificial rubbing stones. The latter cut more quickly, remain of the same consistency throughout, and do not clog or scratch so much as the natural article. In the hands of careless or semi-skilled operatives, the artificial stones are more effective and less likely to do damage.

Cleansing and Washing Down

Whatever degree of rubbing down of old paintwork may be necessary, it is absolutely essential that the surface be perfectly clean before repainting takes place, and a great many paint failures can be traced to neglect of this simple rule. In spite of its importance, there is no doubt that washing down is too often carried out in a haphazard fashion, and that, generally speaking, it deserves very much more care and attention than it receives.

However pure the atmosphere may seem to be, all exposed surfaces must in time collect a certain amount of dirt. In industrial areas this will be obvious, but even in quiet districts there are many factors which constantly contribute to the formation of grime or grease. Coal, coke, or gas fires are responsible for deposits of saline or sulphurous acids. Motor and other forms of traffic add their quota of oily fumes; fingers leave invisible marks on doors, windows, and other places. To paint over them is to invite disaster, and they must be removed if all risk of subsequent trouble is to be avoided.

Cleansing agents commonly employed for washing down paintwork include soda, various forms of soaps, scouring and abrasive powders, decorators’ soap, and sugar soap. Of these, soda is the most widely used – primarily, no doubt, because it is cheap – though it cannot be said to be ideal for the purpose. Since it is an alkali, it combines with grease which it readily removes and so long as it is used in reasonably weak solutions it is relatively innocuous; if the solution is strong, however, it is liable to cause a certain amount of trouble. The average painter is apt to be careless about the amount of soda he adds to a bucket of water and, because the stronger the solution the more easily it removes dirt and grease, there is a natural temptation to use a lot of it. In this form it needs more rinsing off, after it has done its work, than it generally receives, with the result that traces of alkali are not infrequently left on the surface: in the case of exterior paintwork, the latter is, as a rule, somewhat porous and therefore more likely to absorb it. On occasion, this residue may cause blooming of a varnish or enamel, or even mild saponification of the film.

Soaps and soap powders vary so much in composition and quality that it is not easy to express any definite opinion on them. Some contain a relatively high proportion of alkali and must be used with restraint. Others tend to leave on the surface a film, hardly visible to the naked eye, but which dries rapidly and requires a good deal of rinsing to remove; traces of it left on the surface when repainting is carried out may adversely affect the adhesion of the new finish.

Scouring and abrasive powders are more extensively employed by the house-wife for keeping paintwork clean than by the decorator for preparing it for repainting although, in point of fact, they are generally more suitable for the latter purpose than for the former, since many of them are intended principally for hardware, though they may also be recommended by their manufacturers for painted surfaces: they often contain harsh abrasives or strong alkalis, or both, and though, when applied repeatedly for maintenance, they leave the paint film clean and free from grease, they also scratch it, cause it to lose its gloss, and gradually wear it away. The minute scratches which the use of such materials leaves may not show much, but they constitute weak points through which moisture may penetrate to the underside of the coating and weaken its bond.

A good sugar soap or decorators’ soap is unquestionably the best type of cleaning agent to use. It is free from abrasive ingredients and has a detergent action which efficiently gets rid of impurities from most surfaces. It is certainly safer than soda, from the point of view of any possible damage it may do, and it has the additional advantage that the container in which it is packed usually gives instructions on the correct strength of solution to employ for different types of surfaces.

Before interior paintwork is washed down, precautions should be taken to protect with dust sheets any adjoining surfaces which might be damaged by splashes of the cleaning fluid. Polished floors, for example, can be badly damaged by alkaline solutions and the discolorations caused in this way are extremely difficult to remove.

In many cases, where washing down is not effectively carried out, the trouble is due to insufficient changes of water. This is not always the fault of the painter, for on many jobs, especially where exterior work is under treatment, unlimited supplies of water are not always available or readily accessible. The result is that the dirt is not so much removed from the surface as transferred from one part of it to another. Whenever possible, hot water should be used for making up soap, soda, or sugar-soap solutions, and the latter should be applied warm.

Work should always be begun from the bottom upwards; in this way, runs which might leave unsightly tracks can be avoided. Flat surfaces can be treated successfully with a cloth, though care should be taken frequently to change the part in contact with the dirt. A stump brush or old distemper brush is preferred by some men for applying soda or soap solutions, and is essential for getting into angles, quirks, and mouldings, where dirt tends to collect. On flank walls or large expanses the best results are obtained by employing two men, the first to apply the solution and the second to follow on with cloth or sponge behind him. If only one man is available, the main point to bear in mind is that the edges should be kept wet. After rinsing off, the work should be dried off with the leather, which should be soaked in clean warm water and then wrung out until it is nearly dry. While glossy surfaces can be wiped or rubbed without any risk of damage, it is safer, when dealing with flat or eggshell-flat films, to dry them by dabbing with the leather, as if stippling.

When heavy deposits of dirt, sulphurous matter, or other impurities have to be removed, pumice or other abrasive powder is not infrequently used and is usually effective for the purpose: care must be taken, however, to ensure that all gritty particles are rinsed off before the surface is repainted or revarnished; special attention should be given to corners, mouldings, etc., which must be well rinsed out; if this is not done, pumice and other particles will attach themselves to the paint or varnish brush and be transferred to more conspicuous areas of the surface, to the detriment of the new finish.

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