Be careful with the 400 to not get carried away. Sand as little as necessary. Up to 600grit is fine, but I've been playing around with lapping heatsinks for the past couple weeks and 800 grit seems to be the sweet spot for the last noticeable difference. Problem is 600 is the highest my local hardware stores carry so if all you can get is 600, that's fine. If you wanted to go a little higher, I got the higher grit sandpaper from Dave's Lapping Kit. The kit has all you'll need and then some.