These troubles may arise from moisture in the wood escaping after the paint has become hard, but is generally caused by the paint becoming so hard as to lose all elasticity. Change in temperature and the expansion and contraction of the wood or metal, you can readily understand, will cause all these conditions when the paint is no longer pliable. Lead paints, in drying, remain elastic for a long period, and hence, are not so prone to these troubles as zinc and other neutral pigments in which the paint dries hard. It must be said in all fairness, however, that these neutral pigments may not begin to crack or peel, until after a lead paint applied at the same time has chalked very considerably. Under coats of old paint frequently cause cracking and peeling. Putting a coat of paint which dries quickly over one which contains much oil or varnish, and which has not become thoroughly dry, produces cracking.

The use of too much dryer, or too little oil is a frequent cause of paint cracking and deserves special mention. The necessity of killing all knots and resinous or sappy places in the surface of wood, with a suitable coating must be born in mind, as this is a frequent cause of blistering. Raw oil is superior to boiled oil, as it is more durable, less likely to crack or blister and being more penetrating, enters into the pores of the wood better, forming a hard, tenacious filling, as well as coating.

Avoid the direct rays of the sun on fresh paint, as far as possible, as this is a frequent cause of blistering. The priming coat of paint is by far the most important and the best materials should be used for this. Raw oil and the earth colors, such as ochre (previously ground in oil) should be used. The addition of 25 per cent. of white lead will improve the coat. Never use dry ochre. Zinc white is a poor primer, because of its tendency to peel off. The priming coat should not be heavy.