Depends a bit on the final finish you're planning on applying, but I've used 000 fine sifted silica which is cheap, and works (but use a respirator) and 120 grit aluminum oxide. The sand creates more "tooth" so you need to go a little easy on the pressure. The Al-Ox produces a finer texture, but is harder to find and much more expensive.
I use a moly based bake on resin finish and they come out feeling similar to the surface of a pmag. Not mirror smooth like the original USGI... there's a slight texture but they still slip in and out of the magwell very nicely. They also take plenty of abuse and still look new.