Varnish Cracking and Checking

¶ Cracking and Checking. A common defect which is pretty well described by the terms "cracking" and "checking". The varnish breaks up much like a mirror. There are many causes of the defect. Severe changes in temperature when the varnish is not thoroughly dry and hard and the use of cheap, inelastic varnish in thick coats are common causes. Under coats which are not dry when the finishing coats are put on and the use of under coats which are too thick are fruitful causes.

¶ If slow-drying varnish is applied over quick-drying varnish, the finish is likely to crack. The safe method on all surfaces is to use the same quality of varnish from the start to the finish of the job.

¶ The use of two varnishes having entirely different degrees of drying on the same surface is unwise. The use of a cheap first coat varnish and an elastic good quality finishing coat of long-oil varnish invites trouble from cracking and checking or worse.

¶ Some finishers will not use shellac for varnish under coats, even when absolutely pure, on the theory that it is very brittle and prevents the varnish from taking hold of the surface and anchoring firmly. This is open to argument, obviously, since shellac is very widely used, and, when spread on in thin coats and rubbed down considerably, none of it remains except what is lodged in the wood pores as a filler.

¶ The best and only safe method for finishing is one - which puts on an elastic priming coat next to the wood or metal. Then from the primer to flat color coats the elasticity should decrease. From the flat color to the finishing coats the elasticity should increase by degrees. Any other method is likely to result in cracking. This, of course, has special reference to automobile and similar finishing, but the principle is the same for enameling.

¶ Varnish films which are exposed to unusual amounts of ammonia fumes, coal gas, alkalies, etc., are likely to crack.

¶ Varnish which has cracked can seldom be refinished to eliminate the cracks, except temporarily, unless the varnish is stripped off far enough to cut out the cracks, and then refinish.

Next Page: Chipping, Flaking, Scaling, Peeling Varnish.