Wheeler engineering makes a 600 grit paste that is available from Midway, I think its about $14 for a tin of it and it will go a long way. I have several different grades (rated in microns) of industrial diamond paste that I use in die making, but it isn't readily available or cheap. The abrasive paste used in the autobody world is a good really fine paste for lapping parts, it takes a while to remove much but that's what you want. Its really hard to put the metal back on after removing too much. Stay away from valve grinding compound unless you really want to cut the metal away.