Part 1: Grit Sizes, Sandpaper

We'll look at sandpaper, it being perhaps the most likely of stuffs where most of us come across grit sizes, we'll look at other applications a bit later.

Sandpaper, as you'll know, comes in various degrees of coarseness (or fineness?) and is, nowadays, grit stuck onto fairly flexible fabric (you don't see the original grit-on-paper too often nowadays – at least, I don't).

The degree of coarseness is measured in ppi - Particles Per Inch ie the 'p' number - of particles, laid side by side, that fit across 1 inch. A common scale (search, eg, 'Wikipedia Sandpaper') starts at Extra Coarse, p12 and goes down to Ultra Fine at p2500. Personally, I don't think I've used anything coarser than p60 or finer than p1000 and treat those as being extra coarse and extra fine. The common range for woodturning is: p80, p120, p180, p240, p360, p400 then, maybe, p600 and p1000. In addition to all those, I, along with quite a few others, keep intermediate grits of p60, p100, p150, p220, p320, p500 and p800.

Why keep so many sizes? Let's have a look at the effects of sanding and perhaps the reasons will emerge...

For general use by chippies, 120 grit (p120) is usually satisfactory for sanding along (ie with) the grain of softwood skirting boards and the like – the resulting scratches don't show (usually). On hardwoods you may have to continue with another couple of steps or so finer – and if you sand across the grain on any wood, the scratches will show for a few or perhaps many, more steps down from there – and, when woodturning, there will always be some parts of the workpiece that get sanded across the grain.

The idea is to 'smooth' the surface with a relatively coarse paper – quite possibly a step coarser than you think you need. After that, all you need or want to do is to remove the scratch marks with one step finer paper then move to the next finer. I've heard it said that if you, initially, use one grit finer than the job needs, or skip a step, it will take ten times as much work to get the results you want – and I won't vouch for only ten times...

The finer you go, the more work there is to remove blemishes – but the less work there is to smooth out only the previous scratches. This will tell you that the earlier, coarser papers are the important ones.

Depending on the wood you're working with on the lathe, a rough guide could be:

• With your turning chisels, cut your piece as closely as you can to what you want.
• Use p80 for final, minor, shaping (p60 at a push) to remove small ridges and/or rough patches in the grain. If it's still not quite right, either go back to your cutting tool or continue sanding - don't go any finer.
• Step down to the next finer and take off the scratches.
  . Check – are there hills or mountains or hollows or valleys? Step back and look to your turning chisels to be rid of them, as excessive coarse sanding can easily make the situation worse by altering shapes. After tooling, go back to the coarser papers.

. Continue with the next finer paper.

Check – are there still scratches left from a previous paper? Step back.

At some point you'll begin to 'lose' the scratches – perhaps as soon as p240 or so but don't stop there – if you're going to apply a finish of oil or wax or whatever, that finish will highlight even fine scratches.

At around the p300 mark, the piece will probably start to show a bit of a shine. Some people stop at p320 for less special pieces. My own feelings are that it's worth going to p400. For special pieces, down to p1000 shows a nice (blemish-free?) polish.

So. The reason for keeping an extended range of papers is, I suppose, as much one of confidence as practicality – working down the normal range should mean that after the first, coarsest grade, several wipes with each should remove all evidence of the previous – whereas using the intermediate ones in addition, a few wipes with the next one down pretty much ensures it... that's got to be good, eh?

There are other, somewhat more recently developed, types of sandpaper – grits bonded to a (usually synthetic) fabric web, which are more flexible and allow the dust to fall through rather than gather and clog the abrasive. One such (there are others) is called Abranet, and comes in a very similar range. It is, to my mind, better at the job – rather more expensive to buy but lasts longer – and is noticeably more economical in the long run (but don't throw out the conventional stuff).

Articles by Rod

04.11.13 Front Page

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