Many bowl makers use sanding discs in an electric drill to sand bowls. The idea is to start with a coarse grit disc, then use a succession of finer grit discs to remove scratches left by each previous one. Each disc leaves its own scratches, getting finer and finer as you work through the sequence. But it can be hard to tell whether a scratch was made by the current disc or the previous one. If you sand with 120 grit, then go on to 180, which disc are the scratches from? If you go from 120 to 100 or 150 grit, those discs will be more effective than the 180 at getting rid of sanding scratches. But the smaller the interval, the harder it will be to tell them apart.
Sometimes you can overlook the deepest scratches until the last stage, or even until after you have applied the finish. If you miss the deep ones from the coarser disc they can show badly later.
There is a simple method to distinguish between them. A particle of grit on the upper part of the disc will be moving from left to right as the disc spins clockwise. If the part of the bowl being sanded is moving upwards in the lathe, the scratch that particle makes will run diagonally from top left to bottom right. The angle will depend on the relative rotation speeds – running the lathe slowly will make the angle easier to see. If you reverse the rotation for the following disc, its scratches will run from top right to bottom left. When you can’t see any scratches running the other way, that disc has done its work. Reverse the rotation again and go on to the next one.
You can reverse either the lathe or the drill to give this result. You can also switch between the upper and lower quadrants of the sanding disc. Either way, you will be able to spot deep sanding scratches more easily.
Another benefit is that reversing the direction helps remove projecting fibres. This applies also to hand-held sandpaper, which leaves vertical scratches. This method of getting rid of sanding scratches is not applicable to hand sanding.