CRACKING.
It has been said that the natural destiny of varnish is to crack. When a varnish has worn itself out, lost its elasticity, become brittle, it will, despite the best laid plans of men and science, fissure and crack. In so doing it simply responds to a natural law. The cracking that occurs prior to this period of service is of supreme concern to the painter. Probably the greatest cause of varnish cracking—the cause that towers above all other causes—is developed by the hurried system of painting—forcing one coat over another not perfectly dry. Imperfectly dried rubbing coats, or a lack of uniformity in the selection of the varnishes used, often cause cracking. For example, a quick drying rubbing varnish, or a hard drying finishing, even, is employed, over which a slow drying, elastic finishing is used. Antagonism between the varnish coats, or between the varnish and color coats; improperly adjusted foundation coats; exposure to sudden atmospheric changes, including excessive heat; the action of ammonia; poor material—all of these are underlying causes of varnish cracking. Imperfectly seasoned panels or moisture penetrating thin wood panels will tend to crack the varnish used over such surfaces. The cracks in varnish due to a continued straining of the panels are termed "force cracks."
Force cracks are usually found just over the steps on the carriage body, running in long, circular lines, also on the panels under the seat riser, and on the seat riser. The vibration of light, insecurely stiffened carriage bodies is generally a direct cause of premature cracking of varnish. The accompanying cut of a buggy body shows the usual location and sweep of force cracks. This class of surface fissures is very easily distinguished from those due to causes previously mentioned.