Shrinkage involves a process whereby the priming, roughstuff, color, varnish, etc., apparently goes in while the grain of the wood goes out. Graining out is often due to a priming coat that is not given adequate time to dry hard and firm. This soft layer of rather slow drying pigment, if sealed from contact with the air prematurely, is a powerful inducement to grain showing. Spongy, porous roughstuff, deficient in resinous matter and weak in its binding property, is also often responsible for graining out. Good reliable priming, lead and roughstuff coats, allowed to dry thoroughly, each and all of them, arrest the graining out tendency. Improperly seasoned wood is a prolific producer of grained out surfaces.
Moisture confined under a body of paint and varnish is bound to make its exit right speedily, and this it does by voraciously sucking the paint and varnish material in and pushing the grain of the wood out.
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.
SWEATING.
Sweating is the taking on of a gloss after the varnish coat has been rubbed. The principal cause of varnish sweating is rubbing it before it has sufficiently hardened. Varnish laid over a coat of color or of varnish that lacks somewhat of being dry is prone to sweat. When a coat of varnish has been rubbed and allowed to stand for some time—over night, say,—in a close paint or varnish room atmosphere, it will take on a sort of a gloss or greasy scum which comes under the head of sweating. It would be in the highest degree dangerous to permanent or brilliant results to flow a coat of varnish directly over a sweaty surface. The sweat that overspreads a rubbed varnish surface by reason of the absorption of atmospheric impurities can be quickly removed by lightly rubbing with a little rotten stone and water. The sweating out of a surface rubbed before it has adequately hardened can only be remedied by allowing the surface to become hard and then re-rubbing.
DEADENING, SINKING IN, ETC.
This describes a varnish when it goes "flat," loses its lustre, and refuses to shine in the public eye. The causes of this trouble are, briefly; unseasoned timber, imperfectly dried under coats, such as, for illustration, a four-day rubbing varnish surfaced and finished over after permitting the rubbing only two days in which to dry. Porous under coats which absorb too great a percentage of the oil of the varnish cause deadening; and porous under coats, let us bear in mind, produce by far the larger share of varnish deadening.