I've been using two grits for sanding down my abs. 50 to start, then some no name one that's really gritty. I have no idea what is, so I'm not gonna make up a number for it. Once it's nearly level with the rest of the case, I switch over to 300 grit, and I finish with 1000.
Good luck when the replacement case comes in. It looks like it was gonna be win.