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Decorating Defects - their cause and cure

Flaking of Paint

In many instances, the flaking of paint is preceded by and is the inevitable consequence of cracking. Since flaking is the result of lack or loss of adhesion, we will consider in this section the principal causes of this condition, which are as follows:
I. Dampness of the surface.
2. Dirt or grease on the surface.
3. Destructive agencies which develop on the surface after the paint has been applied.
4. Insufficient key.
1. Dampness of the Surface.-It is common knowledge that paint will not adhere properly to damp surfaces and no attempt should be made to apply it to those which obviously contain an excess of moisture.
Unfortunately, the appearance of wood, cement, plaster, stonework, and other structural materials is often deceptive in this respect, for though they may look reasonably dry, they may, in fact, hold too much moisture to make painting safe. This is especially the case with plaster, the face of which is very readily affected by the atmosphere; in a new building, for example, if an interior is well ventilated and the air has a low degree of humidity, the plaster may seem dry both to the sight and to the touch, although actually it may be very much the reverse. If such a surface is sealed too soon, water pressure from behind may push the paint away to such an extent that the film can be peeled off in strips.
Flaking may take place soon after application or may be delayed for many months, depending on the strength of the water pressure which is exerted and on the strength and resistance to moisture of the priming coat. If the latter is 'of an impervious nature, such as is normally possessed by a high-grade aluminium primer, the film may remain unbroken but part company from the surface in the form of blisters.
Paintwork which has dried and hardened in the ordinary way and which has adhered well to a wall will ultimately flake if moisture finds its way into the structure behind the coating-as, for instance, by penetration due to porous brickwork. In such circumstances, the damp will weaken the bond and in time cause cracking and finally flaking.
2. Dirt and Grease.-Similarly, the presence of dirt or grease on a surface before the latter is painted will prevent or interfere with the proper adhesion of the coating; such impurities must therefore be removed by thorough washing or, in the case of grease, by the use of turpentine, petrol, or other suitable solvent. Traces of grease left on the work will, in addition, prevent the paint film £rom hardening properly.
3. Destructive Agencies.-Moisture may well be included under this heading which, however, is intended specifically to cover substances which may not be present, or may not be in evidence, when the painting is carried out, but which may subsequently develop beneath the paint film, with the result that the latter is detached from certain areas and local flaking takes place. They include efflorescence, rust, and resinous exudations.
No precautions can reasonably be taken by the decorator against the possibility of efflorescence, but in the case of rust, careful preparation of the metal and the use of a corrosion-inhibitive primer should be effective. Provided sufficient time and trouble are expended, the risk of trouble from resinous exudations from woodwork can also be reduced to a minimum.
4. Lack of Key.-While the provision of a good mechanical key, such as is provided by rubbing down wlth abrasives, greatly assists adhesion and is most desirable wherever it can be obtained, its absence does not necessarily mean that paint will not adhere properly. It is, for example, quite possible to make paint stick firmly to glass, though the latter is quite impervious and smooth. In such a case, the nature and composition of the first coat of paint is all important.
In repainting old work, flaking of the new finish is often the direct result of insufficient preparation of the surface. In the first place, the painter should make quite sure that the old paint is firmly attached to the latter; if it is not, and new paint is applied over it, sooner or later flaking will occur, the new bringing the old with it.
All loose matter or that of which the adhesion is at all doubtful, should consequently be removed before re­painting. If the existing paint is in good condition, but is hard, as is often the case with old gloss or enamel finishes, careful rubbing down is very necessary in conjunction with a fairly strong soda or sugar soap solution which will etch the surface and provide a key. If a new undercoat is applied without this preparation, it WIll merely lie on the top of the surface without adhering properly and premature flaking will occur.
Flaking will sometimes take place on both new and old work if there is undue delay, from some cause or other, in applying the various coats. Undercoats, on occasion, if left exposed for three or four weeks without the application of a finishing coat, will provide very little key for the latter, unless they are well rubbed down before the final coat is put on.
On, smooth, impermeable surfaces, which cannot be roughened before painting (as, for instance, glass), flaking will soon take place, owing to the absence of any key, unless special care is taken in selecting a suitable primer. The latter should not be of an oily nature but should be made with a medium which is itself of a tenacious nature and thus does not depend for its adhesion on penetration, but on its own viscous character. A good mixing varnish, gold size, or bodied oil is thus to be preferred to raw oil for such a purpose.
In general-especially on non-porous grounds-thin coats of paint are less likely to flake than are thick ones.