when i would wet sand my car it was to make it strait. I would primer it than i would put on a guide coat, since i was using yellow primer, i used flat black as my guide coat. then i would wet sand or "block" with 400 then let it dry primer it again, block again, do this 3 times with the 400 then after that i would go up in grit i would go to 800 then 1000, then paint it. But this was with a car and not plastic pieces