Paint prep! I like finishing the polyester primer in 400 grit (dry). I also like adding a final step before sealer, and that is going over everything with these 3m soft sand sponges. The 'super fine' grit is somewhere between 500 & 600 grit. It takes the edge off of everything nicely, and it also makes any missed scratches show up while you're sanding with them. I like wrapping them around a yellow soft block, just like in the bottom picture.