Machine-sanding operators are usually skilled personnel with a sensitive touch, otherwise edges are easily dubbed over and veneered surfaces cut through to show bald patches of glue which cannot be masked with ordinary stains. Great care must also be taken with hand sanding, which should be regarded as precision work, for the pressure of the hand is inclined to vary with the movement of the stroke, and there is always a temptation to press more heavily over localized defects. At all costs sharp angles and facets must be preserved, or the effect is lost and the work assumes a woolly appearance.
34 Sanding wood surface
Hand-sanding rubbers are usually made of cork, which gives the requisite firmness and resilience, but wood blocks covered with hard felt or rubber are suitable, and also wood alone, although it is inclined to wear the paper through more quickly. In all hand or 'touch' sanding the pressure should be fairly light but firm, as too great a thrust on the block loosens the grit and clogs the surface, while too rapid a stroke tends to skate over and crush rather than cut through the fibres. In other words, the innumerable small teeth of the grits must be given time to accomplish their task. Ideally, the wood surface should be brushed clean of dust ahead of the block for it is the loose grits grinding on their fellows which destroy the cutting action more than the actual fibres of the wood, while the sanding-block should be tapped smartly on the bench from time to time to clear the paper of accumulated dust. A fine wire card is also useful for clearing the paper. In flat work which is to be gloss polished it cannot be too strongly emphasized that the chief difficulty lies in getting rid of the imperceptible bumps and hollows which are never apparent until the mirror finish magnifies them out of all proportion, therefore the blocks should be large and the stroke as long as possible, particularly with scraped surfaces which always tend to bumpiness. Orbital sanders are excellent for this general levelling, as the size of the pad is equivalent to a half sheet of standard paper, and provided the weight of the machine itself is allowed to do the work, without bearing down, good surfaces are obtained. Belt sanders cut faster but are not so accurate, and great care must be taken with veneered surfaces or they soon rub through. Practically all belt sanders are grossly overloaded in relation to the fractional horsepower available, and no pressure must be applied or bumpy surfaces are inevitable, and motors soon burn out. A useful precaution in sanding out to the edges of flat work is to nail waste pieces of similar thicknesses at either end, so that the sanding block or belt can travel through without dipping. For sanding curved surfaces, mouldings, etc., short lengths of thick dowel-rod or broom-handle, or shaped rubbers made out of softwood to fit the curve, should be used, and the time spent in preparing them will be well repaid in the cleanness of the final finish. This crispness is particularly important in chamfered work and in simple nosings where the curve dies away into a flat surface, for any softening of the edges or waviness of outline is immediately apparent.
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