The primary goal of sanding is to produce a surface that doesn’t show machine marks or sanding scratches after you apply a stain or finish. If the scratch pattern can be made even, you may achieve satisfactory results sanding only to #150 grit. Stationary sanding machines are best for doing this, though hand sanding will also work, especially if you aren’t applying a stain.
Sanding is very personal. We apply different pressures, use sandpapers to different degrees of wear, and sand for varying lengths of time. Always sand in the direction of the wood grain when possible. Sanding across the grain or diagonal tears the wood fibers, leaving scratches deeper and more obvious than the scratches left with the grain.
Sanding with your fingers backing the sandpaper will cut away softer spring-growth wood faster than harder summer-growth wood. This will leave ridges in coarse-grained woods such as oak that you may not notice until after you have applied the finish. If you are sanding a flat surface such as a tabletop, and you want to keep it flat, always back your sandpaper with a flat sanding block.
If you could sand just the right amount with each sandpaper grit, it would be most efficient to go through each consecutive grit—#80, #100, #120, #150, #180—and so on. Most of us sand more than necessary with each grit, so you may spend less effort skipping grits. This is especially the case when using machine sanding tools.
No matter whether you sand by hand or with a machine, always remove the sanding dust before advancing to the next-finer grit sandpaper. The best tool to use is a vacuum because it is the cleanest. A brush kicks the dust up in the air to dirty your shop and possibly land back on your work during finishing. Compressed air works well for removing sanding dust if you have a good exhaust system, such as a spray booth, to suck the dust out of your shop.
Before applying a finish, whether directly to the wood or over a previous coat, wipe your hand over the surface to check that it is clean and to remove any small amount of dust that may have settled.