Get a maroon 3M buffing pad. The kind they use in auto-body shops . Chuck up a bore brush in a drill motor or drill press then slip a case over the slightly larger bore brush and hold a 3"X3" section of pad to the case as it spins. Wear gloves. This will break the oxidation on the surface exposing the softer brass under the hardened surface. Don't worry about the micro scratches they will now polish out next loading.