I would not use anything rougher than 120 grit ...with 40 grit the scrathes themselves are in the .015 to .020 range , without my chart handy from work for exact reference ..it is at least that ...a 120 grit leaves abought .005 deep scrathes....an 150 grit is best at round .003 Thousandths.
Just a tip for a good seal after .
