The Technique of FURNITURE MAKING

Synthetic lacquers

These include cold acid catalyst, polyurethane and polyester lacquers, all of which have greatly increased heat, water and spirit resistance, tough films and high gloss, particularly the latter which is extensively used for wireless and television cabinets as there is little if any sinkage, and the gloss is comparable with a vitriol (German) piano finish. Polyester lacquers are, however, unsuitable for small turnovers, for they require dual-feed sprays for intimate dispersal of the catalyst hardener throughout the lacquer, and the resin-dust in surfacing can be an industrial hazard if suitable precautions are not taken.

Acid catalyst lacquers

Originally phenolic resins were used, but modern formulas include urea formaldehyde, melamine and epoxy resins with alkyd plasticizers for increased resistance to wet, heat, etc. A two-pack lacquer is the norm,

547 Portable rotary vane air compressor. (By courtesy of A. Bullows and Co. Ltd)

composed of lacquer, syrup and separate acid catalyst, often hydrochloric acid, which must be mixed with the lacquer in the stated proportions. Setting is by polymerization, but the mixed lacquer has a pot life of several hours and enough can be mixed for the day's work. A one-solution pre-catalysedform is now available with a shelf life of about six months, but the pre - catalysation is obtained by lowering the solids content, and therefore the build-up is less rapid.

Polyurethane lacquer

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.