I used 320 grit because that's what's recommended by Interlux for Brightsides. And you're right - it doesn't get rid of the brush marks - at least with the foam cushion on my shimmy sander. Soon as the rain lets up, I'm going to try the shimmy sander with a piece of formica behind the paper. 320 grit takes out the brush marks just fine when wet sanding with a wood block, but I'm getting too old to tolerate that much pain.