From what has been said on the subject, it will be seen that the painting of new plaster and cement is attended by a considerable amount of risk and there is no doubt that the longer it can be postponed, within reasonable limits, the better. It is clear that in addition to the possibility of chemical attack on a film of oil paint, there is danger of failure from the comparative impermeability of a paint film which prevents the normal drying out of the surface.
It follows from this that if any form of decoration is to be applied to new walls it should be one sufficiently porous not to interfere to any extent with the normal evaporation of the water from the surface. Materials which suggest themselves for the purpose are size-bound distemper, washable distemper, and certain types of flat oil paint, the choice depending primarily on whether a purely temporary coating or one which is more durable is required.
Size-bound distemper comes under the first of these two categories, but has the advantage that it is cheap to apply and easily washed off after a year or so. Oil-bound distemper does not, as a rule, contain sufficient oil to make it liable to alkaline attack, though cases of the latter are not entirely unknown when this form of decoration has been employed. Casein-bound distemper is alkali-resistant, and is therefore quite suitable for the purpose. As a general rule, only lime-proof pigments are used in distempers and there should not therefore be much risk of discoloration. In most cases, both oil-bound and casein-bound distempers will prove reasonably durable in bedrooms or living-rooms where atmospheric conditions are not abnormal, but cannot be expected to stand so well in kitchens or bathrooms where there is nearly always a good deal of condensation.
Flat oil paint provides a more durable form of coating than distemper or water paint, and certain types give a film which is porous enough not to impede the drying-out of the plaster. There is rather more danger of alkaline attack, but this will depend largely on the proportion and nature of the vehicle. Flat oil paints which contain ingredients of a waxy nature as matting agents should be avoided in this connection, since the films they give are less permeable. Full-gloss paints or enamels should not be used on new plaster or cement surfaces until the latter have been given plenty of time to dry out; even after twelve months, it is sound practice, if this type of finish has to be used, first to apply one or two coats of a special alkali-resisting primer, as a precaution against chemical attack.
Since asbestos sheeting contains a substantial proportion of Portland cement, it presents, when new, similar difficulties to the painter as do ordinary cement or lime plaster work. The treatments advocated, therefore, are much the same – to allow the sheeting to stand unpainted, or with a temporary coating of distemper, for several months, until the alkali it contains is no longer capable of being activated or, alternatively, as soon as the material is reasonably dry, to coat it with a special alkali-resisting primer which will either neutralise or resist alkaline attack or, in some measure, combine both these functions.
It must be remembered that asbestos sheeting is a porous material which can absorb moisture from any damp surface with which it is in contact, and if it is to be used for any work in which it may conceivably do this the back and edges of the sheeting should be painted with a good damp-resisting paint. A good many cases of the paint coating on the face of the asbestos being discoloured or saponified are due to failure to take this precaution, which should also be observed when the sheeting is used for exterior work.
A form of pre-treatment often recommended for new asbestos sheeting before painting is the application of a solution of zinc sulphate, using about 1- lb. to a gallon of hot water. This is applied hot, allowed to dry, and then well rinsed with warm water. A fibre brush should be used for the application of the solution, which, owing to its poisonous nature, should be handled with care.
It is claimed that, by this means, the alkali is effectively neutralised, and this is more likely to be the case than when a similar wash is applied to cement or lime plaster because the amount of alkali contained by asbestos sheeting is limited and there is no risk, as with cement or plaster walls, of fresh supplies being continually brought forward from the backing. At the same time, the process cannot be regarded as a reliable one.
Stucco work consists of a mixture of Portland cement and sand and consequently, when new, the same considerations apply as when cement is being painted. It is always desirable to allow surfaces of this kind to stand for at least a year before being painted, and even after this period has elapsed a coat of alkali-resisting primer should be given.
Previously untreated stucco work which has stood sufficiently long to ensure that the alkali is no longer active can be painted in the ordinary way, but in view of the porosity of the surface it is better to avoid the use of raw oil since there is the risk that too much of this will be absorbed, leaving the film underbound. For this type of work, the primer is best made up with boiled oil which, being more viscous, is not absorbed to excess.
The decorator is not very often called upon to apply an oil paint to interior brickwork, for when surfaces of this nature have to be treated, they are usually finished in a distemper or washable water paint. Occasionally, however, a more permanent coating is required, and in this case a good lead paint will probably be found most satisfactory. Care should be taken, however, that the work is quite dry, for bricks are capable of absorbing a large quantity of moisture. In the case of new brickwork, oil paint should not be put on until sufficient time has been allowed for the lime in the mortar to become inactive and a coat of alkali-resisting primer may be advisable.
The porosity of many types of bricks and their ability to absorb and retain moisture make it important, when outside brickwork has to be painted, that this should be done only during a spell of dry weather. Owing to this property of absorption, the vehicle of the paint should preferably be of rather a viscous nature, for if it is too thin, there will be too great a tendency for too much of the medium to separate from the pigment, leaving the latter underbound, so that the coating will have little resistance to the weather. For this reason, as a general rule, boiled or bodied oil is to be preferred to raw oil.
One of the difficulties sometimes experienced in painting brickwork is that the suction varies on different parts of the surface, so that some areas are more liable to flake and break down sooner than are others.