Painting Plaster and Cement

If plaster, cement, and surfaces of a similar nature were always dry when they were required to be painted and could be guaranteed to remain in this state indefinitely, their decoration would be a simple matter, and would involve little more than adjusting the consistency of the primer to the porosity of the surface and following on with the finishing coat or coats in the ordinary way. Unfortunately, in new buildings, they are frequently not in this condition, and this fact is a constant source of trouble to the painter. The problem they present is not a new one, but although a great deal of research has been done on the subject no completely satisfactory solution has as yet been found.

Water is undoubtedly the villain of the piece, and this fact cannot be over-emphasised. This is not to say that it is solely responsible for the decorator’s difficulties, which may also result or be added to by unsound plastering practice, inadequate or unsuitable painting specifications, and insufficient knowledge, both of the materials he is using and of the precise nature of the surface on which they are employed.

It is safe to say that a great many painting failures on new plaster and cement are due to circumstances beyond the control of the decorator, in that he has to carry out his work on grounds which are not fit to receive paint; the situation is complicated by the fact that a superficial examination of these surfaces may give no indication of their condition. It will help to explain the principles on which their treatment should be based if the main causes of these failures are enumerated; they are:

Chemical action, resulting in the softening of the paint film or its discoloration, or both.

Lack or loss of adhesion of the paint.

Defects in the surface, originating from poor-quality plaster or bad plastering technique.

Efflorescence.

Chief Types of Plaster

Before the reasons why one or more of these forms of failure occur can be properly understood, it will be necessary first to outline the nature of the chief types of plaster or cement surfaces likely to be encountered.

Portland Cement

This is used in concrete, cement renderings, asbestos sheeting, and lime plaster (which is sometimes gauged with it). It consists of about 60 per cent, lime, 20 per cent, silica, 10 per cent, alumina, and smaller amounts of oxides and alkalis: these alkalis are soluble and combine with the lime freed upon setting to form caustic compounds which, when activated by moisture, will attack the oil in the paint. Because of this, there must always be considerable risk of painting the surfaces mentioned above, unless they are thoroughly dry.

Lime Plaster

There are considerable variations in the types of limes used in this plaster, some containing substantial amounts of soluble alkalis and others very little. To make it set more quickly, the plaster is often gauged, either with plaster of Paris or with Portland cement: if the latter has been used, there is an obvious danger of paint failure, but even if it has not, and the lime used is relatively free from alkalis, there is still the possibility that the plaster has been contaminated with alkalis from bricks, mortar, or backing.

Calcium-sulphate Plasters

These are sometimes known as gypsum plasters, because most of them are made from the mineral of that name, although for some another mineral, anhydrite, is used. There are two main types, the hemihydrate or quick-setting type, and the anhydrous variety.

When gypsum is heated at a moderate temperature under suitable conditions, it loses about 75 per cent, of the water it contains, leaving the substance commonly known as ‘ plaster of Paris,’ which is the basis of the hemihydrate type. Plaster of Paris (often referred to as ‘ gauging plaster ‘ or simply ‘ plaster ‘) sets so rapidly on being mixed with water that it is, by itself, unsuitable for ordinary plastering purposes, but the setting can be delayed by adding certain substances, such as glue. In this form, such plasters are known as retarded hemihydrate plasters. A considerable number are sold under various proprietary names (e.g. ‘ Adamant,’ ‘ Gothite,’ etc.).

When the gypsum or, alternatively, the anhydrite, is heated at still higher temperatures, so that all the water content is driven off, the residue forms the basis of the anhydrous variety; plasters of this type differ according to whether they are lightly, moderately, or hard burnt, and according to the subsequent treatment to which they are submitted in the course of manufacture. Unlike the hemihydrate group, the anhydrous do not set quickly of themselves when mixed with water, and alum, zinc sulphate, or other substances are added to accelerate the setting. Though some of the lightly burnt type take on an initial set fairly rapidly, the hardening process is comparatively slow with most of them. Anhydrous-type plasters, too, are marketed under a number of trade names (e.g. ‘ Sirapite ‘ (lightly burnt), ‘Statite’ (moderately burnt), ‘ Keene’s,’ ‘Parian,’ etc. (hard burnt).

The majority of calcium-sulphate plasters, both of the hemihydrous and anhydrous varieties, are ‘ neutral,’ in that they do not, of themselves, chemically attack the paint film. But it is fairly common practice, in using them, to add lime in order to make them work more easily and when this is done, they are potentially injurious to the paint. It must be remembered, too, that though the setting or skimming coat of plaster, to which the paint is applied, may be itself innocuous, soluble alkalis from the backing may be drawn in to it.

Moisture Content: Although it is common knowledge that considerable quantities of water are used in such building operations as bricklaying, cement rendering, and plastering, few people outside the industry realise how great these quantities are. figures quoted by Llewellyn and Eldridge * show that, at a rough estimate, average brickwork will contain about 30 gal. of water per cubic yard, when first laid, and that fairly dense concrete will have a similar amount. Portland cement rendering f in. thick will contain about a gallon a square yard, lime plaster nearly half a gallon per square yard in each coat, and skimming coats of calcium-sulphate plasters about a quart per square yard. Thus, when a building is new, the amount of ‘ free ‘ water may total several tons, and it is calculated that in the brickwork and plaster of an ordinary room 16 ft. X 15 ft. X 9 ft. there may be about 1 ton.

So large a quantity of water must take a considerable time to disperse even under favourable drying conditions. Painters are often anxious to know how soon it will be ‘ safe ‘ to proceed with the decoration inside a new building, but it is quite impossible to answer this question in general terms. The kind of weather experienced during the building operations, the nature of the bricks, cement, plaster, and other materials employed, the ventilation of the rooms, whether or not any artificial heating has been used, the kind of decoration required – these are only some of the factors which must be taken into consideration, and they must necessarily vary from job to job.

One thing, however, is quite certain. If the plaster is coated prematurely with a more or less impervious film, such as is provided by paint, the natural drying-out process will be interfered with, for the plaster must be allowed to ‘ breathe ‘ if it is to harden properly and retain its essential properties. Before any form of coating is applied, the surface should, ideally, be air-dry, 1.e. its moisture content should be the same as that of the atmosphere in the room. In practice, it seldom is given time to reach this condition, but, so long as its excess of moisture is relatively small, no great harm is done. The difficulty to the painter lies in judging its approximate condition, for, as has been said, appearances are deceptive. In dry weather, the plaster in a well-ventilated room may be bone-dry to the touch, but actually contain too much water to be decorated without risk; conversely, in damp weather, it may seem wetter than it really is throughout its thickness.

Since moisture within the structure is the main cause of trouble, it may be asked why some kind of waterproofing treatment should not be given to the plaster to seal it, and the decoration applied on top. Quite apart from the difficulty of finding any kind of material at a reasonable price, which would fulfil this purpose for more than a short period, the moisture thus held back would either prevent the plaster from hardening or bring about its disintegration, for gypsum is slowly dissolved by the action of water and, for this reason, is unsuitable for exterior exposure. 1. Failure Due to Chemical Attack

The first form of failure which we have to consider in connection with the painting of new plaster or cement is that due to chemical action. The surface may appear to be reasonably dry and oil paint applied to it may dry and harden in the ordinary way; within a short time, however, parts of the film become soft and sticky; in addition, there may be watery blisters and oily runs and perhaps bleaching or discoloration of the coating; in mild cases, the trouble may be limited to peeling in parts due to the fact that the underside of the film has been softened by alkaline salts and has thus lost its hold on the plaster.

The salts in question may be present in the plaster, if this contains Portland cement, or may be brought forward from the backing. Their effect is to saponify, or turn into soap, the oil in the paint, and to discolour the pigment portion if this happens to be one, such as Prussian blue or chrome yellow, which is not proof against them. It should be stressed, however, that alkaline attack on the paint film will not take place unless moisture is present to activate the alkali, so that if the wall is dry there is no risk of paint failure from this source. Moreover, although free lime is mainly responsible for this form of trouble, it is only when it is accompanied by sodium or potassium salts that saponification of the paint takes place.

Of the oils used for paints, linseed is particularly susceptible to this kind of attack, so that unless the plaster or cement is known to be dry, paint made up with linseed oil should not be used. China wood oil is more resistant to alkaline action but not sufficiently so that a coating of which it is the vehicle can be applied with any degree of assurance on this kind of surface.

The safest way of avoiding trouble is undoubtedly to refrain from painting new plaster or cement until the free lime it contains is rendered inert by the natural process of carbonation; this begins as soon as the surface is exposed to the air, but it takes many months before any appreciable depth is neutralised. For this reason, it is often recommended that no more than a temporary coating, such as that provided by distemper, be applied to new walls for a year or so since, being porous, this does not interfere with the drying-out process of the plaster and contains little or no oil (according to the type of distemper employed) liable to attack. Unfortunately, it is common practice to specify a more or less permanent and impervious finish, such as oil or gloss paint, for application to the wall sur-faces of bathrooms or kitchens in new houses, within a few weeks of the plasterer having completed his work; in such cases, the painter’s problem is what treatment to give so as to reduce to a minimum the risk of premature failure.

There are two alternative processes adopted with this end in view. The first is to try to neutralise the free lime by chemical treatment and the second to seal the surface pores with a material resistant to alkaline attack, so as to provide a buffer coating between the plaster and the subsequent decoration.

Neutralisation

In the first process, since the objective is to counteract the effects of an alkali, the use of an acid comes readily to mind, and those recommended for the purpose include vinegar (acetic acid), a solution of zinc sulphate, and various others. Although some of them – notably zinc sulphate – are claimed to be effective at times, they cannot be described as reliable remedies. The weak point is that, except possibly to a chemist, the amount of free lime present is an unknown quantity and consequently how much neutralising medium is required is also unknown. If an excess of the latter is applied the effect on the paint film may be just as disastrous as that of the free lime. Moreover, the treatment affects only the alkali on the surface and fresh supplies may continue for a long time to be brought forward in solution from the backing, as the wall surface dries out.

Sealing

The form of treatment more generally advocated is to apply a special alkali-resisting primer which most of the leading paint manufacturers can supply. There are various formulations of this, but many of them rely on the use of a China wood oil and resin-acid vehicle. Such paints are capable of offering considerable resistance to alkaline attack, but it should be borne in mind that they have their limitations; to expect them to prove successful if they are applied to a wall far too wet to receive them is unreasonable. The most that can be said of them is that if new plaster or cement has to be painted after a few weeks or months, these primers offer the best available safeguard against trouble from chemical action. 2. Failure Due to Lack or Loss of Adhesion

One of the chief difficulties in painting new plaster and cement is to ensure proper adhesion of the paint film. If, as so often happens, the surface is coated before the wall structure is sufficiently dry, the pressure of water sealed up behind the paint may lift the latter in the form of blisters. If it is not strong enough to do this, it may collect in the surface layer of the plaster and cause it to disintegrate to a certain extent, so that the paint will not bond properly. Again, as previously explained, mild chemical action may also bring about softening of the underside of the paint coating, so that peeling may ensue from this cause. In other cases, loss of adhesion may result from efflorescence which, however, will be dealt with later in this post.

An additional reason against painting surfaces of this kind until there are good grounds for assuming that they are dry enough for the purpose is that sealing up the surface with an impervious coating like paint may have a bad effect on the plaster itself and prevent it from completing the hardening process. In tests carried out at the Building Research Station a few years ago on new plaster which had been painted, it was found that the thickness of the soft upper layer of plaster corresponded roughly with the depth to which the linseed oil in the paint had penetrated; this suggested that the penetration of the oil may interfere with the hydration of the new plaster.

On plaster which is moderately porous, there is usually a certain amount of texture to assist adhesion, but on the hard-faced variety, such as Parian or Keene’s, this is often not the case. Excessive trowelling gives them a smooth, polished face which has very little absorption and on which it is not easy to make distemper or even paint adhere properly. Once such plasters have set hard, it is very difficult – and expensive – to impart a mechanical key by rubbing down on large wall surfaces, neither is it particularly desirable that this should be done, since it is liable to interfere with the surface structure and the uniformity of the suction.

Following the Trowel

To avoid this difficulty, the practice of what is known as ‘ following the trowel ‘ is usually adopted. This involves the application of a ‘ sharp ‘ coat of white-lead paint immediately after the plasterer has finished work and as soon as the plaster is strong enough to withstand the action of the brush. Since there is a good deal of misunderstanding about this form of treatment, it should be emphasised that its sole purpose is to promote adhesion of subsequent decoration: it does not, as many painters seem to think, enable further coats to be applied while the wall is still wet.

In following the trowel, there are two essentials to be observed. The first is that it must be done just as soon as the plastering is completed, so that the particles of wet paint can be drawn into the surface pores of the plaster. If there is too much delay, there is not sufficient suction to draw the film into the plaster, and it is far better not to attempt it at all. The second requirement is that the paint should be really ‘ sharp ‘ – 1.e. that it should contain a minimum of oil. Traditionally, a white-lead paint is used in following the trowel, and it is quite sufficient to thin white-lead paste with turpentine or white spirit to brushing consistency without adding any additional oil.

It should be added that not all the experts approve of the system of following the trowel, though of recent years some at least of them seem to have modified their opinion on this point. On the whole, it can be regarded as sound practice and, although it certainly does not ensure success, it is unquestionably of practical value in some instances. 3. Plastering Defects

Some of the troubles which arise in the painting of new plaster are due not to errors of omission or commission on the part of the decorator, but to those of the plasterer. There is room for a great deal more co-operation between members of the two crafts than there has been in the past, and there is a tendency on the part of the plasterer to forget that any shortcomings on his part will make the decorator’s work more difficult. It is true that there is a great variety of proprietary plasters on the market which differ to a considerable extent in their working properties and setting times, so that, no doubt, the poor quality of some of the plasterwork encountered is attributable to the fact that the plasterer has been required to work with materials unfamiliar to him, but he is in the happy position of knowing that his mistakes will be covered up by the decorator.

For example, it has been already explained that plaster of Paris is often mixed with lime putty to speed up the setting and reduce the risk of shrinkage; when plaster of Paris is mixed with water, it may be worked for only a short time before setting begins; consequently, a finishing coat of plaster which contains it must not be over-trowelled. If it is worked too long after the setting has begun, there may be a local or even a general loss of strength.

One of the most common defects in plasterwork is that known as ‘ pitting,’ which occurs at times in calcium-sulphate plasters to which lime has been added, and also in lime plaster, and shows itself in the form of small pits or craters on the surface: these are caused by the presence in the mix of particles of unslaked lime which, after the main mass of the plaster has hardened, combine with water and expand, causing flakes of plaster to break away in the process. The difficulty about this defect is that it may, and usually does, occur after decoration has been applied to the surface, so that its prevention is beyond the power of the painter.

Failure of the plaster to harden is invariably due to the presence of too much moisture which may come from the backings or from long exposure to moist conditions. In such circumstances, the decorator is not at fault, but he cannot be held blameless if the excess is directly due to the surface having been painted prematurely, so that the natural drying-out process of the plaster is impeded.

On the other hand, if the moisture evaporates too quickly before the setting of the plaster has been completed, the latter will have a tendency to be short and friable – a condition usually referred to as ‘ dry-out.’ This may be the result of too much water having been absorbed by porous backing coats or, more occasionally, by the work having been done in hot weather; it can also be induced by injudicious use of artificial heating to dry out the surface, and this risk should be borne in mind when such a course is recommended. ‘ Dry-out’ may be so slight as to be almost unnoticeable when the painter starts work, but it will probably mean that premature flaking of the paint film will take place; in this event, the back of the flakes will usually bear particles of plaster.

On occasion, plaster affected by ‘ dry-out’ undergoes what is described as ‘ delayed expansion,’ and exhibits cracks and ridges. This may occur if the plaster, after drying, absorbs moisture which renews the setting action.

From this brief summary, it will be seen that most of the defects which plasterwork develops are due to causes beyond the decorator’s control, though in one or two instances they can be produced or aggravated by incorrect surface treatment. 4. Efflorescence

A fourth form of failure of paintwork on new plaster or cement may be brought about by efflorescence, which manifests itself in the form of a whitish crystalline deposit either beneath the paint film or on top of it.

Most of the materials, such as bricks and mortar, used in constructional work contain varying amounts of soluble salts. As walls dry out, these salts are carried in solution to the face of the plaster where, when the water evaporates, they remain as a crystalline powder; even small amounts of salts can produce quite a heavy deposit. The salts may pass through the paint film (or through a distemper coating or wallpaper, if either of these materials are used instead of paint); in this event, they can generally be removed without much difficulty by scraping and little harm is done. Alternatively, they will form underneath the paint film and detach it from the surface by flaking, accompanied at times by small eruptions on the surface of the plaster. Some types of plaster themselves contain salts which may give rise to this defect, but in the majority of cases the salts originate from the backing.

The most serious feature of efflorescence is its liability to recur so long as soluble salts are present in the structure and there is sufficient moisture in it to dissolve them and carry them forward. In consequence of this, the longer decoration can be delayed, the better. New plaster and cement should be carefully examined for traces of the deposit and, if any are found, they should be removed and the surface kept under observation for a time to see whether they renew themselves. It is better to remove them by dry scraping since, if they are washed off, part of them will be dissolved in the washing water and will probably reappear on the plaster as soon as the water has evaporated. Efflorescence may occur after an interval of years on a wall surface which has never previously shown any signs of it, if soluble salts are present within it and the wall becomes damp – as, for instance, from a burst pipe or the rain driving through brickwork or mortar which has become porous.

There is no effective way of preventing the appearance of these salts if conditions favour their formation. No attempt to neutralise them by chemical treatment should be made or the cure may prove worse than the disease. Nor is it feasible to apply any coating to seal them up, for they are capable at times of forcing stout material like Lincrusta, or even glazed tiles, away from the wall. The only course is to continue to remove them as they appear until either the source of supply is exhausted or there is no longer sufficient water present to bring them forward.

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