The isocyanate cured polyurethanes were at one time toxic but are now controlled and are outstanding for adhesion, flexibility, gloss, water and chemical resistance. Originally in two-can form with separate hardener, they are now obtainable in one-can pre-catalysed solutions. The usual application is by spraying apparatus, but they can be brushed more easily than cellulose lacquers, and pad-application french polish type polyurethane lacquers are available for pure hand methods.
Both acid catalyst and polyurethane lacquers are very suitable for dining-table tops, etc. and a full gloss polish is possible with the use of appropriate grain fillers, sealers and base coats, etc. flatted with wet and dry silicon carbide paper moistened with either white spirit or water (the latter gives a fiercer cut, and the surface should be lubricated with a little common soap), followed up by cutting-down pastes and burnishing creams. They can also be used for thin finishes showing the grain, with the dust nibs cut off with 7/0 abrasive paper, or steel wooled and waxed. The secret of applying these lacquers is to complete the required number of coatings as quickly as possible, for if each coat is allowed to set glass hard then any build-up will lie as separate layers, and any chance cutting through of the top layer in sanding will show up as a blue blaze. With rapid coating (three in one day) each layer tends to soften up the under layer, and some degree of amalgamation is achieved.
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