If you want to go back down to unfinished metal, removing all the color that is currently present, a mild acid should do the trick. Try vinegar, wipe it own and rinse it back off with warm water, then lightly oil. Also try Coca Cola, don't laugh, it contains phosphoric acid. A bit sticky with all that sugar, but it'll clean off easily. If these don't take it off you'll need a stronger acid, or try steel wool.