You will need to epoxy the voids on the top deck and the mag end. I like to sandblast or use a 1/8" burr to roughen up the area to be epoxied. Anywhere epoxy will be in contact with the cases needs to be roughened and spotlessly clean.
I like jb weld as it goes nice and runny with a bit of heat. If you have already ground through, use a bit of gaffer tape to block the holes.
You can go further, don't be scared to cut into the epoxy. I epoxy before grinding. That way I can use the epoxy that the grinding exposes as a guage to get uniformity between ports and how far to go.
40 or 60 is fine as a final finish. If you want a very sexy finish, give it a quick sandblast last.
You can get those handheld sandblasters on ebay for relatively cheap. They are fine for small and infrequent use. Just wear goggles and dress like an Arab when you use one.