I've used the edge of copper 'wheat' pennies and Klean Bore Formulae 3 to remove surface rust from blued surfaces. Any coppper color wipes right off. Rotate the penny's edge as it wears. The penny is softer than the blue finish, so it does not/will not mar. Keep the surface wet when doing the rubbing. You can safely use some pressure, but no need to go overboard, rather keep a fresh edge forward.

I have not had occasion to use the technique on a case colored finish.