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.

Yes, sandpaper. You must remember to use very fine-grit sandpaper, and the porcelain must remain wet at all times.

Barely recognizable as sandpaper, the superfine category, which can go up to a 600-grit paper, is used to remove surface blemishes on finishes and sometimes tone down an undesired sheen.

You must remember to use very fine-grit sandpaper, and the porcelain must remain wet at all times. As long as the porcelain is wet, the sandpaper will "float" on the surface and will not scratch it.