A Brief Treatise on Sanding

What's more boring than sanding? You take a part that's sharp, rub it against some sandpaper until the sharp edge is gone, and that's it, right? That's what I thought, until I was introduced to The Theory of Sanding.

It goes like this: Sanding is a process based on a simple concept - you want to scratch the surface repeatedly, making smaller scratches every time. Sandpaper scratches a surface. A specific grit of sandpaper will create scratches that are a certain size and depth. Lower number grits (80, 200, 400, etc) are more coarse and larger numbers are finer (1000, 1500, 2500, 3000, etc). In order to achieve a high polish, you need to work your way up from coarser grits to finer grits.

Okay, maybe not that kind of grits...

What's happening while you sand? Let's say you start with 200 grit paper; you create scratches that are wide and deep (relatively speaking) as you sand the surface. Once you've fully sanded the surface it is covered in these grooves of a particular size. No matter how much you sand with 200 grit paper, you will still have scratches at this size. You can't get a perfect mirror polish with a coarse sandpaper alone.

So you need to go to a finer grit paper, say 800 grit. The 800 grit paper takes down the 200 grit scratches and leaves grooves that are narrower and shallower. Finally, you go to a very fine grit like 1500. With 1500 sandpaper, you remove the 800 grit grooves and leave very small scratches. At this point you can continue to go finer and each time you will be leaving smaller and smaller grooves behind; eventually they will be so small that your surface will look and feel mirror-smooth.

But why not just start with the finest grit sandpaper, if that's the groove size we really want? You certainly can, but depending on how the surface looks at the beginning, that might be a very, very slow process. If your surface is relatively rough compared to the desired finish that you want, it will be much faster to use a coarser sandpaper to remove most of the roughness. Once you've eliminated the original surface roughness, then you can work your way up with finer sandpapers that create the desired surface finish.

Fun fact to end with: if you need extremely fine polishing compound, toothpaste works great! It has small particulate embedded in the paste that is meant to gently polish your teeth. (Just don't think you can go the other way and substitute jeweler's polish for toothpaste.)