Sandpaper coarseness is determined by the size of grit on the paper—ranging from extra-coarse to extra-fine. Using sandpaper that's too coarse could damage the material you're working with.

Then, it’s off to sanding with 200 and 600 grit wet sandpaper. Slowly, the soldermask is removed and the top copper layer appears. After that, it’s just a matter of sanding and scanning ...