Special care must be taken that both the plaster and the wall itself are perfectly dry before they are painted.
There are several methods of applying the paint, all of which are influenced by the very absorbent nature of the plaster.
The plaster may be primed with glue size to prevent absorption, and then four coats of ordinary lead paint applied. Care should be taken that the whole surface stands out with an equal gloss, after which it may be flatted.
The plaster may be primed with two or three coats of boiling linseed oil. When this is dry it is covered with a thin coat of weak size, tinged with red lead, to stop all absorption, and give the work a uniform appearance, and then finished off with two coats of oil paint, and a flatting coat if required; or with two coats of coloured varnish, as described at page 197.
Another plan is to prime the plaster with white lead and linseed oil containing a little litharge, and mixed to the consistency of cream. When the oil is absorbed into the plaster, and this coat is dry, another similar coat is given. In a few days a third coat may be added, rather thicker, and containing a little turpentine. By this time, the plaster being thoroughly saturated, a fourth coat, thinned with equal parts of turps and oil, may be added, and then the flatting coat; or, when the work is not required to be very durable, the fourth coat may be omitted.