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Decorating Defects - their cause and cure

Non-drying of Paint

The failure of a paint film to dry and harden, or its liability to resoften after it has apparently dried in normal fashion, may be due to one or more of several causes, some of them exceedingly complex and able to be under­stood only by those with considerable knowledge of chemistry. For our purpose, it will be sufficient to discuss the problem as it appears to the decorator and to outline the main conditions which may prevent or interfere with the ordinary drying process.
Non-drying may originate from the ground or surface over which the paint is applied, the properties of constituents of the paint itself, or from agencies exterior to the paint film. These causes will be dealt with in this order.
1. Non-Drying Due to the Underlying Surface.-It is common practice to ascribe the drying of oil and gloss paints to oxidation, that is, by the oil content of the paint absorbing oxygen from the air, converting the oil to a tough, leathery substance known as linoxyn. This is true in the ma~, but the process is to some extent assisted by the evaporation of the volatile solvents, such as turpentine or white spirit. Both oxidation and evaporation can be retarded or influenced by various factors affecting the groundwork, notably by the presence of dirt or grease.
Grease.-Careful preparation to remove impurities of this kind­especially grease-is therefore essential before painting or varnishing. Special attention should be given to areas where it is most likely to be de­posited, as, for example, in kitchens or on doors, cupboards, and other surfaces normally subjected to a good deal of handling. In small kitchens and sculleries, in which paint or distemper has peeled badly, the plaster thus exposed may be impregnated with grease from cooking fumes, and it may prove very difficult to get it clean. The presence of grease is more likely to cause trouble on impervious surfaces such as glass or metalwork, where a little spreads a long way; thorough washing with a warm solution of sugar soap, followed by careful rinsing and leathering off before the paint is applied, will usually prove sufficient on most groundwork, but on glass or metal, a wipe over with a rag moistened with petrol or turpentine, immediately before painting or varnishing, is a wise precaution.
Wax.-A good deal of trouble is caused by the presence of old wax polish on paintwork, wood and other surfaces, especially since it is not always easy to detect when this has been employed. Ordinary washing down will not remove such deposits and the use of a solvent is necessary. In some instances, particularly when varnishing is carried out on such surfaces, the finish may appear to dry satisfactorily but may subsequently become soft again. This is due to the fact that skin-drying only has taken place and the film has failed to harden right through; the solvent in the varnish dissolves the wax left on the surface but the wax prevents it from evaporating in the normal way. Later, the solvent which has thus been trapped rises to the top of the film, softens the latter and forces its way through, bringing with it wax in solution. This forms on the top of the varnish and prevents the film from drying again.
Trouble is sometimes experienced when a surface coated with certain types of egg-shell or flat varnish, in which wax is employed as the matting agent, is being painted or revarnished. In other types, in which aluminium stearate or some other wax-free substance is used for this purpose, there is no risk of the new finish failing to harden, but it is as well to regard all old flat or semi-flat varnished surfaces as suspect from this point of view. To distinguish between the two types, rub the surface briskly with a soft rag; if wax is present, the old varnish will gloss up, but if some other flatting material has been used, it should not take on any added lustre.
Wax in Paint Solvents.-Another potential cause of non-drying is lack of sufficient care in the use of paint and varnish removers of the solvent type. Many of the latter contain a proportion of wax which serves a double purpose: it prevents the mixture from running too freely on vertical surfaces and it also hinders the volatile solvents from evaporating too quickly. Consequently, after old paint or varnish has been stripped by means of a remover of this type, the surface should be wiped over with a cloth wetted with petrol or turpentine, in order to dissolve any wax which may have been left, after the solvent has done its work. Failure to observe this precaution may mean that the new finish will not dry properly.
Alkali in the Surface.-Paint or varnish applied to new Portland cement, lime plaster, asbestos sheeting, and similar surfaces, may remain permanently soft and sticky, or may revert to this condition after having apparently hardened. This is due to the action of the alkali, in the form of free lime in the surface, which saponifies the oil content of the finish. This form of defect is dealt with at length in the section dealing with the priming of new plaster and cement.
Strong alkalis which attack new paintwork in this way may also be introduced by the painter himself, if he uses a caustic type of paint remover to strip old coatings and fails to rinse the surface thoroughly before repainting. Strong solutions of soda or sugar soap, employed for washing down old work prior to repainting, may have a similar effect, if any traces of alkali are retained by the surface, due to inadequate rinsing.
2. Non-drying Due to the Paint Constituents.-The second possible cause of paint failing to dry and harden properly may lie in the -properties or constituents of the paint itself. Faulty technique, due to carelessness or ignorance .of the nature of the materials used, is mainly responsible when this occurs.
Abuse of Driers.-In particular, the misuse of driers is frequently to blame. These products are added to paint to accelerate the absorption of oxygen and do so effectively, if they are used with discrimination. If too much driers are added, however, oxidation takes place too quickly with the result that a skin is formed on the film and this prevents the process from continuing normally throughout the thickness of the coating, so that the lower part of the latter remains permanently soft.
Many painters add driers recklessly because, unfortunately, the strength of these substances varies not only according to their nature but according to the formulation of individual manufacturers, so that it is not possible to say just how much should be incorporated in a paint. In addition, the amount must vary according to the atmospheric conditions; in cold weather, for example, considerably more can be used than on warm days. All that can be said on this point is that they should at all times be employed sparingly and that it is far better to add too little than too much. Any excess may prevent the film from hardening properly and, even if it does not have this effect, it will probably affect the durability of the coating.
It is hardly necessary to add that driers should never be added to any ready mixed paint unless any such addition is authorised in the manufacturer's instructions-which is seldom the case. The remarks above, there. fore, apply only to the decorator who knocks up his own materials, and the best advice which can be given him is to purchase a good brand of driers from a reliable firm, note their effects under varying conditions and, if they prove satisfactory, use no other.
Over-thick Films.-Just as the addition of too much driers may prevent a paint film from hardening normally, so may the application 0 coatings which are too thick, and for a similar reason. The outer or weather side of the film skin-dries and thus does not allow the necessary supply of oxygen to reach the lower portion. Such coatings may remain soft for a very long time and, in extreme instances, permanently. This form of defect is less likely to occur with paint than with varnish films, the latter being rather more impervious than the former.
Drying Properties of Different Pigments.-The addition of drying agents to a paint~must be regulated to some extent by the nature of the pigments employed. Some pigments, such as red or orange lead in particular, white lead, and, to a lesser degree, the siennas and oxides of iron, exert a definite drying influence on linseed oil. Others, notably zinc oxide, lithopone, titanium oxide, and antimony oxide, are neutral, i.e. they neither hasten nor retard the oxidation of the oil. A few, such as carbon black and Vandyke brown, actually slow down the process. It is important, therefore, that when such pigments are employed in a paint, the proportion of driers which is incorporated should be adjusted accordingly.
3. Agencies Exterior to the Paint Film.-Deviation from normal drying of paint and varnish films may originate also from agencies which have nothing to do with the surface over which the coating is used, or with the nature of the film itself, but which are mainly due to atmospheric conditions. Dry air, with a moderate but regular circulation, is the most satisfactory from the painter's point of view. When the weather is cold, or the air is charged with humidity or is stagnant, the evaporation of the solvents and oxidation of the oil are slowed down to a greater or lesser extent and the coating takes longer to dry. This fact should be borne in mind; on outside work the decorator is more or less at the mercy of the weather and little or nothing can be done to improve painting conditions, but inside a building it is often possible by the exercise of a little trouble and forethought to do so. In churches and chapels, which are frequently kept closed for the greater part of the week, the air-particularly near the ceiling-is sometimes so stagnant that unless measures are taken to procure adequate ventilation, paint or varnish may remain soft and tacky for days on end.