Home >>Professional Decorating>>Decorating Museum>>Flaking of Distemper

Decorating Defects - their cause and cure

Flaking of Distemper

Although the flaking of distemper can often be traced to bad workmanship on the part of the decorator or to conditions over which he has little or no control, it should first of all be pointed out that, owing to its nature, this type of material is more likely to flake in the ordinary course of events than is paint. It contains a smaller proportion of binder and consequently less tensile strength than paint; the pIgment is coarser and the material is applied in thicker coatings which have comparatively little and often no elasticity.
Broadly speaking, when premature flaking of distemper occurs, the most likely causes are one or more of the following:
I. Poor atmospheric conditions in the room in which it is used.
2. Application to a surface which provides no key and has little porosity.
3. Application over a surface which is too absorptive.
4. Application over an existing coating, the adhesion of which is weak.
1. Atmospheric Conditions. Flaking of distemper coatings within a relatively short time after application is more liable to occur in interiors where there is a good deal of condensation. It is true that there is rather less risk of the latter with walls finished in distemper than with painted surfaces, because distemper is more porous and provides a rather" warmer" surface than paint, but in bathrooms and kitchens, where a good deal of steam is normally generated, a certain amount of condensed moisture is bound to be deposited on the walls and ceiling, unless the ventilation is much better than is usually the case.
The presence of this moisture will not be so evident as it would be on more impermeable coatings, but its effect is destructive; the alternate wetting and drying of the film, as the moisture condenses and later evaporates, weakens the binder of the distemper and sooner or later induces peeling. The use of distemper in rooms where such conditions exist and cannot be remedied should therefore be avoided so far as is practicable.
2. Lack of Key.-This condition is most generally found on hard­faced plasters, particularly when they have been trowelled to a smooth finish and which, owing to the density of their texture, have very little absorption. In, some cases it is practically impossible to use distemper, with any prospect of it remaining intact on surfaces of this nature for more than a few months, unless some preliminary coating is given to help adhesion. It is impracticable to obtain a mechanical key on large areas of plaster by the use of abrasives, but most manufacturers supply special priming sizes for the purpose.
Alternatively, if an oil-bound distemper is to be employed, a preliminary coat of the petrifying liquid advocated by the manufacturers of the distemper can be given. A third alternative is a thin coat of gold size in which a little dental-grade pumice powder may advantageously be incorporated, to supply the necessary" tooth." The use of a lining-paper is also to be recommended.
3. Overporous Grounds.-Distemper applied directly over a ground which is too porous will soon flake, because the water in the material is absorbed too quickly and leaves most of the binder on the surface instead of penetrating the latter and obtaining a key. The suction of the ground, in such cases, must first be satisfied by the use of a preliminary coat of clairecolle, in the case of a size distemper, or, if a washable distemper is to be applied, by a coat of the petrifying liquid recommended by the makers of the distemper.
There is less excuse for the failure of a distemper applied directly to a ground which is too porous than there is when flaking is due to insufficient absorption and lack of key in the plaster. On the latter, the material brushes on easily and smoothly and the finish will probably look quite satisfactory for a time. Should there be too much suction, however, the fact can be readily detected by the drag of the brush and the difficulty of applying the distemper without leaving visible brush-marks.
4. Application over Existing Coatings.-A large proportion of cases in which distemper flakes occur when the material is applied over already distempered surfaces, without removing the existing coatIng.
This has already been discussed and it is only necessary to repeat here that although new distemper can often be quite successfully applied over two, three, or even more old coatings, there is always a risk of failure taking place, even though the existing distemper may appear in sound condition and firmly attached to the wall; unless signs of flaking are clearly visible, the appearance of the work gives no reliable clue to the strength of its adhesion.
Each fresh application of distemper weakens the adhesion of that already on the wall or ceiling, because the water in the material tends to affect the bond, and the drying is accompanied by shrinkage which exerts an outward pull on the old coating. Whenever costs permit, therefore, all old distemper should be stripped before renewal; if this is too expensive, carefully scrape or brush off all loose matter and bind down, either with a coat of thin oil paint, a coat of equal parts of varnish and oak varnish, or a coat of petrifying liquid. On no account should glue size be employed for this purpose. Then follow on with the distemper.