Sounds like the parts are very dirty and no amount of polishing will return the part to its original shine. Maybe hit the parts with a wire brush or green scotchbrite pad. I've been using an ultrasonic cleaner for years, and almost all crud is removed prior to polishing. If the part is zinc plated (90% of metal parts on a pinball are zinc), you can remove the old zinc and replate. If they are fiddly parts etc, just send to me I will re plate during my next replating "session" !