I've got a nagging problem that's just getting worse. My trailer's galvanized rims have turned from their original bright finish to a dull black. I have 4-wheel SS disk brakes, it is surely disk brake dust that seems to be baked into the metal. I've tried 409 and Simple Green with little to no effect. Before I get out the bigger chemical guns like acetone, laquer thinner, etc, any ideas from others? Thanks in advance.