Mobil 1, a synthetic oil. Slather it on the rust spots. Keep using either someting softer or, yes, the copper penny or a round tip. The rust will keep getting softer as you work away the surface. When there is no longer any red rust you have gone as far as you can go. Someone said to submerge it in Mobil 1. Phoey. The rust doesn't know if there is a silly millimeter or a foot of Mobil 1. So just slather it on.