Fire-cracks (which in some parts of England are called air-cracks) in plaster walls should be treated before giving the primary coat of paint with a coat of weak glue size (1/4 lb. best Scotch glue to 1/2 gal. water) applied when the size is quite hot. About 1 sq. yd. should be done at a time, and the size should be wiped off at once with a piece of old rag, the object being merely to fill all the small cracks with size. The surface of the plaster should be carefully wiped, for size should never be used on a plaster surface except for the purpose of filling cracks.