I think that most of your respondents don't understand that you are using the lapping compound on the the end of your apex tip to get a better grip on the screw heads. While their suggestions for heat and penetrant are good ideas, they just aren't applicable to your situation.
I'm holding a 16 oz. can of 600 grit. Part number is 4A 51804. The side of the can lists grits from 36 up to 1200. 400, 500, and 600 are listed as very fine.
800, 1000, and 1200 are extremely fine. I would suggest something in the 400 grit area.
10,000 years ago we would have eaten you. Today, we drag you along and allow you to pollute the gene pool.