ABRASIVES

Old-time woodworkers gave their jobs a final smooth finish by rubbing them with all sorts of materials — dogfish skin, for example. Now modern abrasives have taken over; sand isn’t among them, but we still use ‘sandpaper’ and ‘sanding’ as general terms.

Of course, smoothing wood isn’t the only job that calls for sanding. In decorating, it helps shift dust stuck in paintwork, remove excess filler from cracks in walls, and much more besides. Fine sandpaper can equally well be used to smooth a matt surface — such as that of natural timber— and to take the shine off a glossy one, such as varnish (often in order to help a subsequent coat to stick).

Sanding can also be used for minor wood-shaping operations, such as rounding off sharp corners; but the emphasis is on ‘minor’. It’s too slow for removing large amounts of material. If you don’t believe this, try sanding out a scratch from a piece of wood. In any case, other tools are much better suited to the job — planes, rasps and so on.

But, of all the different abrasives on the market, which should you use? It depends on what you’re sanding and the surface you want.

All abrasives — except for steel wool and various harsh metal and plastic discs — consist of a backing coated with grains of ‘grit’. Grits can be crushed glass or garnet, or modern synthetic materials. The latter are more efficient, longer-lasting, less likely to become clogged, and able to cope with a wider range of materials. Unfortunately, they also make the abrasives more expensive.

But choosing the right kind of grit isn’t everything. It’s also essential to pick the right ‘grade’. A large, coarse grit will cut faster than a small, fine one, but also leave a rougher finish. For this reason. Most sanding operations need at least three different grades: a coarse one to remove most of the material, a medium one to remove any residual material, and a fine one for the final finish.

Sadly, however, abrasive manufacturers don’t often use simple terms like ‘fine’ and ‘coarse’ to describe their products. They use a numbering system; and the snag is that it varies with the type of abrasive. With glasspaper you talk about 00 (the finest grade) and 3 (the coarsest). But with silicon carbide paper, for example, you deal in hundreds.

What’s more, similar numbers may not mean the same thing. Silicon carbide 100 is a lot coarser than aluminium oxide 100 (another type). All you can do is look at the paper and decide for yourself. If in doubt, ask your supplier for advice.

The type of backing also matters. The commonest is paper, and it’s usually more than adequate. But some power-tool abrasives use cloth: and tungsten carbide uses a steel backing.

Then you have to consider the way the grit is stuck on. Is, for example, the adhesive waterproof? The most common waterproof abrasive, ‘wet-and-dry’ silicon carbide paper, unlike glass- and garnet paper, can be lubricated with water without falling to pieces — a very useful property. And how much of the backing’s surface is actually covered with grit? For abrasives described as ‘close coat’ it’s between 70 and 100 per cent, so they’re faster and more durable. ‘Open coat’ abrasives, on the other hand, are cheaper and less prone to clogging.

You can sand either by hand or by machine. Electric orbital, drum, disc, belt and floor sanders are all available, each for a different type of job. You fit them with replaceable sheets, belts or loops of abrasive, usually aluminium oxide. Belt and floor sanders are large and expensive, but can readily be hired.

Machines are faster than hand work, and a great help over large surfaces, but they must be used with care — and, above all, kept moving. It’s all too easy to make unwanted hollows, and to rub away details such as sharp edges that you want to keep. Besides, machines won’t go into awkward angles or overcomplex mouldings. In such situations the only answer is to work by hand.

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