RE: cleaned an engine in prestone and it blackened ... what to do ?
You can use a brass tumbler for polishing ammo brass, you have to take everything apart and be sure all the sand is cleaned out of absolutely everything before reassembly. Depending on what polishing sand you choose, you can polish to a mirror shine or rough metal. You can use walnut or pecan shell instead of sand. This will only remove the oxidation but none of the metal itself.