I clean old metal(antique dog licenses and old brass store tokens)by polishing them with an extra super fine buffing wheel.Can't remember the exact fineness...something like .0001 or maybe .00001,and it takes off almost all of the crud but does not damage the metal at all.I picked mine up used a few years back from a bottle collector who used it to clean haze off of his bottles.It never left a mark on the glass.It was pricey but man...it does the job very well.
The patina comes back in no time and you have yourself a nice clean surface.
Unless it is just on the surface though,it is tough to get rid of rust. Once it gets in the metal and starts to pit...nothing much you can do except live with it.