[theastronaut]

I would start with 2000 grit on the smallest, or easiest to repaint panel. Who knows how many coats are on it, you might burn through quickly if it only has a couple coats.

For buffers, you'll want to go up to ~5000 grit if you're using a orbital, and you'll want a long throw for more cutting action.

I prefer a rotary buffer after wetsanding, with a wool pad and Jescar Correcting Compound, then a light swirl remover polish.