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Coloring Finishing And Painting Wood
251. Disadvantages of Lacquer-Finishes.—The user of nitrocellulose lacquer should be informed about the difficulties of applying it, and about some of the shortcomings of the material as a finishing-medium. The chemical composition of lacquers prohibits their use under certain conditions.
The novice can use paint, shellac, and varnish by placing coats of any of them over or under, and no serious chemical reactions are ordinarily produced; but with lacquer one must carefully consider what is to go under or over it, or serious trouble may follow.
The chief disadvantages of lacquer-finishes, in the opinion of experienced finishers, may be listed as follows:
1. Lacquers are very difficult to apply with brushes, because all such finishes set and dry too quickly.
2. Clear lacquers show a tendency to shrink in drying because of absorption by the undercoats, or on account of evaporation of the high-boiling solvents, which are the last of the volatile elements to leave the film. This shrinking makes the problem of the filling up of the large cell openings of very porous woods exceedingly difficult.
3. Undercoats of varnish, enamel, oil, wax, or paint cannot be used directly under lacquer-coatings, because some of the solvents in the lacquer are the same as, or similar to, the liquids in paint-and-varnish removers; consequently they will soften and destroy such finishes. Covering the undercoats with shellac of standard strength, or even with a considerably thinned mixture, usually, will protect them from the destructive action of lacquer-solvents.
4. Lacquers should not be applied directly over surfaces that have been finished with linseed-oil stains or ordinary wood-fillers, because of bleeding and streaking into the upper coats thru the action of the solvents upon the linseed-oil in the undercoats. Under lacquer-finishes, waterstains, and special wood-fillers containing the minimum of linseed-oil should be used. A coating of shellac over a surface filled with ordinary wood-filler will, to a large extent, protect it from damage by the lacquer-solvents.
5. Spirit-soluble stains cannot be used satisfactorily directly under clear lacquer-finishes without danger of muddy effects.
6. Lacquer-finishes must be rather thin when applied; consequently they have a reduced body or depth, and a greater number of coats is required.
7. A high gloss similar to that obtained from rubbingand polishing-varnishes cannot be obtained from lacquers. The highest gloss can be secured by giving a final coat of clear lacquer; but, unfortunately, such a transparent film cannot be applied over pigmented lacquers for outdoor use because of the destructive action of the ultra-violet rays of summer sunlight, which causes serious checking of the noncolored finishing-coat.
