BTW, sandpaper does not get finer with use, only duller. Instead or cutting, dull sandpaper burnishes.
I thought about that before I more or less agreed with the poster who said it got finer. I thought about how they might make sandpaper, and decided that what's stuck to the paper is mostly the biggest pieces for the grade; there are none bigger, but there might be a lot of finer particles. Then I decided there were probably a lot of smaller pieces, too. The big pieces would break off first and the finer pieces last -- at least that's my guess. So...after a while, the "big" 1000 grit boulders slough off the page, leaving carborundum dust that was mixed in during the grinding and sorting operation. Then again, it might well be totally uniform bits of a certain size, in which case it merely grinds slower and slower.
Possibility two: Filing would start to bury the particles beneath nail dust, with their tops poking out the top like minute mountains in fog. This would make them a finer grade, would it not?
I would definitely doubt that the grains become duller. though. That would be like butter making the knife dull.