When the surface of a coat of paint, that is to say, of any under coat, appears rough, especially in the case of patches in old work that have been retouched, the paint, when dry, should be rubbed down with fine glass paper until the roughness has disappeared. All loose paint, or paint that appears loose round the blister-marks, should be scraped away with a knife before the putty is put on. For cleaning old greasy smoke-stained paint limewash or limewater may be used. This kills the smoke or grease, on which no oil paint will ever dry and harden. Some will put a coating of weak size over the smoke and grease; the paint will dry on this, but it is very likely that it will soon crack and peel off.

It is not desirable to keep loading on coat after coat of paint on old work. It is better, when the incrustation caused by successive coats of paint has become very thick, to remove the paint entirely and begin de novo. There are various modes of removing paint. The professional painter will do it by the agency of heat, applying a flame to the surface of the paint; the heat soon softens the color, and it may then be scraped away with a knife.