I would make sure you are extra careful NOT to sand your mating surfaces, like Ice racer said scotch bite and only a couple passes with that can screw up a case half to case half mating surface.
You can also use 3 bond 1211, I use it on both sides of the gasket all the way around but I always use a new gasket when I put a fresh top end back together. It does need 24 hours to set up and dry but that stuff is the best I've found for sealer. Ultra Grey works good also if you're just spotting it where the case halves come together.