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Decorating Defects - their cause and cure
Chalking
Chalking may be described as the liberation, in the form of a powder, of particles of pigment from the surface of a paint film, owing to the perishing of the, oil or binder. In a properly formulated and applied paint, it is a gradual process, operating exclusively from the weather side of the paint film and resulting in the progressive decrease in the thickness of the latter.
Of all the various ways in which an exterior paint coating can fail,
chalking is certainly the least objectionable. Apart from the question of 
appearance, the value of a paint film on outside surfaces lies in the protection it provides against the weather and, if it is to remain effective in this respect, it must retain its continuity unbroken. A paint which, in course of time, becomes hard and brittle loses its flexibility and ultimately develops cracks which, even though at first they are microscopic in size, provide points of entry through which moisture can seep, attack the film from underneath, weaken its adhesion, and finally bring about flaking. When repainting becomes necessary in such a case, it may be essential to strip the coating, thereby adding considerably to the cost of the job, or, at least, to carry out extensive making-good, which will also be expensive.
On the other hand, if the paint film is of the type which wears by chalking, it certainly becomes thinner as the months go by, but since it remains unfractured, it keeps its protective character longer. If neglected too long, it will, of course, become so thin and porous that it will offer very little resistance to moisture penetration, but even so, its renewal will be a far cheaper and easier task, for this will not need to be burnt off; in most cases a good wash down to remove dirt and loose dust will be all that is wanted before the surface is repainted and will generally afford a good ground for the new paint.
Renewing badly Chalked work.
As already mentioned, if only slight chalking has taken place, it will usually be sufficient simply to wash down before repainting.When, however, the defect is very marked and the
old surface has a great deal of suction, allowances must be made for this by increasing the proportion of oil in the undercoat or, in extreme instances, by applying two undercoats, the first containing about as much oil as would a primer for bare woodwork.
Natural Chalking.-It must be recognised that chalking will also occur at times when good-grade materials are employed and skilfully applied on carefully prepared grounds. The causes are not always clear but it is known that combinations of certain pigments and certain vehicles are more prone to this defect than are others. At one time it was considered that the pigment alone was to blame but research has shown that this is not necessarily the case. A paint made on a titanium-oxide base, for example, with raw
linseed oil as a vehicle, will usually chalk rapidly on exposure, whereas if certain types of synthetic-resin media are substituted for the oil, the trouble will be far less pronounced.
As a general rule, white basic pigments chalk to a far greater extent than do the coloured pigments. Again, paintwork exposed to sea air is usually more affected than paintwork inland. The defect- is probably due to the action of certain rays of light; interior paintwork is rarely affected in this way and the same paint, which is immune from chalking when used inside a building, may chalk badly when employed for outdoor work.
Prevention.-While it is difficult to ensure that chalking will not take place in any circumstances, much can be done to minimise the risk of its occurrence, first by proper preparation of the surface and the use of undercoats which are adequately bound, and secondly by the careful choice of finishing coats.
If oil media are used, the addition of a pigment which, when mixed with a vehicle of this kind, shows little tendency to chalk, to one more liable to the defect, will usually decrease the incidence of chalking in a substantial degree.
A properly balanced combination of zinc oxide with titanium oxide, for example, will provide a base for a paint which should give good service. The presence of the zinc oxide will counteract the natural inclination of the titanium pigment to chalk, while that of the titanium will minimise the risk of cracking which a paint film based solely on zinc oxide for its pigment content usually involves.
Again, the mixture of about 75 per cent. white lead and 25 per cent. zinc oxide for a finishing paint on outside. surfaces at the seaside will be less likely to fail by
premature chalking than will a paint based on 100 per cent. white lead. If circumstances permit, the addition of a proportion of coloured pigment to the white base will still further reduce the risk of chalking taking place.