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Furniture Finishing

Checking is recognized when irregular lines appear in a finish coat upon drying, and is one of the various degrees or phases of "crazing," "alligatoring," "hair lining," etc. Inevitably a quick drying, over a slow drying or heavy coat will check, as well as any finish over a partially or top dried undercoat or even a single excessively heavy varnish coat, the top surface of which must necessarily dry first. In all cases the fundamental cause is an unequal rate of expansion or contraction in which the film of the least plastic element must give way to the extent that change in temperature is moderate or severe. Checking may also result after years of service from disintegration of the life of the varnish or in a relatively new finish by expansion of the wood following absorption of moisture. Lacquer is not likely to check over lacquer but may hair line over poor shellac, or of course, where specially made to ''crackle."

Chipping describes a condition in which the finish flakes off the undercoat and is usually caused by coating varnish over too heavy a shellac coat or applying too many shellac coats. The varnish chips off because it cannot knit to the slick, glass-like shellac, especially where sanded very smooth. A finish that is not sufficiently elastic will also chip under hard usage.

Crawling designates a condition in which a finishing material will not completely cover the surface but crawls away and contracts at certain places. The cause may be a greasy, wet, waxy, excessively glossy surface or the application of a heavy material in a cold atmosphere. The remedy in each case is obvious.

Drying differs in its nature as between various types of coatings. The spirit varnishes or lacquers dry by evaporation of the solvents while varnishes or oil enamels dry by oxidation of the oils they contain, with the help of certain driers. Drying of varnishes, fillers and enamels is hastened by dry heat.

Grain raising takes place when a water stain is used in wood finishing, and the wood fibres are raised by absorbing water. These can be stiffened by coating the surface with a thin solution of shellac and when dry sanding off with fine garnet paper, or the wood may be sponged before staining with warm water, allowed to dry and then sanded, but it is risky to sand on the stain itself if the fibres again rise.

Greening is a condition in which the finish appears murky, due to too strong a stain application or the filler not being cleaned off thoroughly. This is also known as "Bronzing."

Lifting or Raising takes place when any finishing material applied over an undercoat has an effect on it similar to that of varnish remover. This is most often noticed where nitro-cellulose lacquers are applied over varnish or paint coats. It may be avoided by purchasing lacquers adapted for coating over varnish or paint, or by using undercoaters that are not readily affected by lacquer solvents.