Terry is correct. 600 grit should be fine.

Get a piece of steel rod of about the same diameter as your bolt and practice on it. Do it once leave it set over night and look at it again to see your mistakes. It may take several trys to get it right. If you do not like what you see on your first attempt take 400 grit paper and erase your work and experiment some more.

It is much less expensive to mess up the rod than your bolt.

Good luck and lets see some pictures of your new pride and joy.