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Decorating Defects - their cause and cure

Discoloration due to the Groundwork.

The third type of discoloration with which we have to deal is that which originates from the surface or groundwork on which the paint is applied; it is probably fair to say that the greater proportion of instances of discoloration are due to this cause and that, when they occur, it is nearly always because of the failure of the decorator to take adequate precautions against them.
"Bleeding" Colours.- The term" bleeding" is applied to the effect produced by certain colours, mainly reds, when another coat of paint or distemper is superimposed. The liquid portion of the new finish dissolves the colour in the old, with the result that, if the former. is of a paler hue, it becomes tinged. The trouble may occur over a coating which has been standing for many years, and is extraordinarily persistent, the application of further coats being insufficient to obscure the discoloration. The writer has known instances when as many as half a dozen coats of white paint have been brushed over an old red finish without success, the last coat assuming a definitely pink hue. All red-painted surfaces which have to be refinished should therefore be regarded with a certain amount of suspicion, and it is a wise precaution to make a test on an inconspicuous portion of the work with the material which is to be used for repainting. To be on the safe side, two patches should be painted, thinning the new finish in one case with linseed oil, and in the other, with turpentine. If, after these have been allowed to stand for four or five days, no discoloration appears, it may be assumed that the old finish will not" bleed," and painting may proceed in the ordinary way. If, on the other hand, there are indications of" bleeding," a sealer coat must be applied. Pure shellac knotting-particularly if two coats are used - is probably the most satisfactory material for this purpose; alternatively, it is claimed that a coat of aluminium paint, the flakes of which form a protective metal film, may be used as a sealing coat; the efficiency of this, in preventing" bleeding," will depend on various factors as, for example, the quality of the aluminium powder, the nature of the medium with which it is mixed, and the age and hardness of the coating over which it is applied. Aluminium powder mixed with a quick-drying varnish is usually successful for the purpose. The addition of the powder to a shellac varnish will probably give maximum protection and is recommended.
It should be remembered that even a very small proportion of a red liable to bleed may be sufficient to cause discoloration in a white or pale­coloured paint superimposed. For this reason, precautions against it should be taken when recoating not only red or pink grounds, but those of an apricot, peach, or similar shade, in case a red stainer, which may be soluble in the oil or turpentine of the new coating, has originally been used. In rare instances, moreover, distempers or even wallpapers may contain red pigments or dyestuffs which may affect oil paints applied over them and may need to be sealed before the paint is put on. Occasionally, too, red or pink distempers or water paints are tinted with colours which are soluble in water and will consequently bleed into a white or pale distemper if used on top of them; in the latter case, it is safer and more satisfactory to strip the existing coating rather than to try to seal it with shellac, the application of which may induce flaking.
Sometimes bleeding may take place when there is little reason to expect it. It is not unknown, for instance, for cheap wood primers to be made from some inert white pigment, tinted with a dye to resemble the traditional white- and red-lead mixture, and this dye may be soluble in the medium or thinner of the finishing coats. The best way of ensuring against trouble of this kind is to use only good-quality primers made from materials obtained from firms of established reputation. , Surfaces originally treated with creosote and other coal-tar derivatives or with bituminous paints will also bleed and discolour light-coloured paints applied directly over them. The use of a shellac varnish, especially when reinforced by the addition of a small proportion of good-grade aluminium powder will serve to prevent this trouble. When costs allow, two coats of the sealing coat should be given.
Rust Stains.-Discoloration of paintwork over iron or steelwork may be caused by the corrosion of the metal and will be accompanied by the rupture and flaking of the paint film. The importance of removing all traces of rust before a ferrous metal surface is painted cannot be exaggerated, though it is often far from easy to do this by means of the somewhat primitive tools and media, such as files, wire brushes, scrapers, and abrasives, which, as a rule, are all that the average decorator has at his disposal for the job. It is no less important that, as soon as the surface has been cleaned, a good rust-inhibitive primer should be applied.
Although we are here discussing discoloration originating from the surface over which the paint has been applied, it may be permissible, in dealing with the subject of rust, to mention stains on paintwork due to the drippings or runs of rusty rain-water from adjoining fittings. Gutters placed above painted stucco, brickwork, weatherboarding, or window frames are frequently responsible for this kind of trouble, often owing to neglect in keeping them in a good state of repair or to the perfunctory way in which they are painted. Overlapping joints, for instance, should be completely coated to prevent corrosion. Another point to which attention should be given is the fixing of iron fittings on the out1ine of a building over paint­work; all metallic surfaces which come into contact with the paint film, as well as screw holes and the screws themselves, need careful priming to keep them from rusting. Yet another potential source of trouble, so far as rust stains are concerned, is wrought-iron door furniture, strappings, or studs. It should be an obvious precaution to see that all such metalwork is free from rust and well protected by paint to prevent it being attacked by corrosion, yet how seldom this is done! The removal of the stains is not easy but may sometimes be accomplished by means of hydrogen peroxide; an effective method of using this is to take a pad of clean white blotting­paper, moisten it with the peroxide and apply it to the affected area; it may be necessary to keep it in place for about a quarter of an hour and to re­moisten the pad.
Discoloration Due to Alkaline Action.- We have already considered the type of discoloration which takes place on paintwork which is subjected to alkaline fumes in the air, but that due to the presence of alkaline matter in the surface under the paint film must not be overlooked. This is likely to occur when new plaster, cement, asbestos sheeting, and other surfaces containing free lime are under treatment; in such cases the discoloration may be accompanied by a softening or stickiness of the paint coating.
Strong alkalis, liable to cause subsequent discoloration of the paint, may be introduced by the painter himself into the surface; the use of caustic paint removers is a case in point. Unless the washing down, after these products have been employed, is extremely thorough, traces of them may be retained in the groundwork, to attack the new coating. Even a strong solution of ordinary soda or sugar soap, used for cleansing surfaces prior to painting or repainting, may bring about a certain amount of discoloration of the new paint, should the latter contain pigments which are readily affected by alkali. To prevent this from taking place, rinse off well after such solutions have been employed and add a little weak acid-e.g. vinegar-to the rinse to help neutralise any residual alkali.