I'm afraid nothing will work... No brasso, no cola, no soda, nothing. Yes, it will be clean, and you'll remove successfully even the hardest dirt, but you'll never be able to remove all the tarnish. And that's because the pollutants that are in the air and the sweat that's left on the machine heads from the fingers, eventually, after many years, get through the pores of the nickel coat, sit on the metal and cause its corrosion. It's irreversible...