The Technique of FURNITURE MAKING
Dulling methods
Contemporary finishes usually call for an eggshell, matt or semi-matt surface in preference to a choked-grain full gloss. Most lacquers are available either as full gloss (burnishing lacquers), matt or semi-matt finishes, the matt appearance being obtained by incorporating silex powder of other additives to scatter the light. These matt finishes do, of course, cloud the wood to some degree, and in fact all dulled surfaces cannot be expected to show the clarity of a high gloss which acts as both mirror and magnifying glass. The after-dulling of gloss surfaces for an egg-shell gloss effect can also be done with finest grade steel wool, which must be skilfully applied to be effective, or with french - polishers' pumice and petrol/gasoline. A handful of water-floated pumice powder (or fine silex powder) is stirred into a cupful of petrol and swabbed over the surface, keeping the mixture constantly agitated. Any degree of dulling can be achieved by using a special soft dulling brush kept exclusively for the purposes, and sweeping the brush backwards and forwards as in painting until the petrol has evaporated and only the dust remains. Silex powder need not be graded, but all powdered pumice should be water-floated by stirring it into a large bowl of water, allowing a little time for the larger grains to settle, and then decanting the liquid, letting it stand overnight and pouring off the clear water. The degree of fineness of the floated powder will depend on how long the coarser grains have been allowed to settle before decanting, and the process can be repeated several times if necessary.
Satisfactory egg-shell gloss surfaces can only be achieved by carrying out to a full gloss polish and then dulling, and there are no short cuts. The dulling of thin finishes with unfilled grain should not be attempted with the above method or the surface will show white deposits.
Spraying equipment
Production-work uses air or airless sprays in vacuum exhaust or water wash booths, etc. but small portable spray outfits are obtainable which will support one gun with pressures up to 60 lb per sq. in (4.218 kg/km2), and sufficient for all but the heaviest materials. These can be the compressor type or fully portable rotarv vane type (547), relatively inexpensive and efficient in use. Light pattern spray outfits for the handv - man are hardly suitable for serious work, as there must be sufficient pressure and feet cube (m3) air delivery per minute fully to atomize the spray or extensive spitting, blobbing, tear runs and orange-peel effect will result, and rubbing down will be as difficult and as tedious as with brush-work. Some form of moisture extractor should also be incorporated in the compressor, or as a separate unit installed in the pipeline, or 'blooming' will result. Even with fully efficient moisture extraction—and the very act of compressing air squeezes out the moisture always present—some blooming must be expected on humid days. This can be controlled by adding anti-chill thinners to the lacquer, which retard the evaporation and thus allow the suspended moisture to escape before it is trapped by the setting film, or by over-spraying the lacquered surface with thinners, but the gun must be good for this and the application skilful. Regulations regarding the storage and use of inflammable materials are fairly stringent, but nonregulation materials are available which are not subject to the legal requirements. Small outfits in which the air is delivered direct from the compressor unit to the gun do not need periodic testing, but air receivers, i. e. storage tanks for air under compression, require safety checks at regular intervals. In estimating for spray work it should not be forgotten that on average one - third of the material is lost.
Wood finishing production methods
Present-day techniques include spraying, hot lacquer spraying, airless spraying, dual-feed spraying for polyester, electrostatic deposition, roller coating and reverse roller coating for sanders and sealers, curtain spraying for gloss finishes and roller printing of wood patterns on chipboard and hardboard. Methods are geared to flow-line production to yield printproof surfaces in the shortest possible time, and call for sophisticated techniques and equipment which become obsolescent almost before they are installed. The small tradeshop cannot hope to keep pace with this mounting tide of new ideas, new approaches and new materials, and it
548 Spraying-booths. (By courtesy of Aerograph deVilbissCo. Ltd)
will continue to use traditional methods, hand- polishing and simple spraying, and taking advantage where it may of worthwhile developments. In the meantime, the perfect finish has yet to be evolved.
In conclusion it should be stressed that no matter what method and what material is used the final polish will only be as good as the ground it is laid upon, for no amount of after finishing can ever disguise poor workmanship and shoddy materials. The thick, heavy gloss of cheap furniture is there for a purpose—it masks the imperfections which a clear finish would only magnify. An illustration of this is the superlative finish achieved by the professional gun-stock maker. The completed walnut stock is sponged with clean water to which a little oxalic acid has been added, dried rapidly with a blow-torch or over a gas-ring and sanded immediately. This process is repeated several times with an occasional rubbing over with very fine pumice powder to burnish the wood and seal the grain, and the wood surfaces can be further burnished by 'boning', i. e. rubbing with a block of the same wood. The resultant surface is dead smooth and velvet to the touch, moreover any subsequent exposure to damp conditions will not raise the grain.
