Don't know how high a polish you want, but Bar Keepers Friend and a green scrubbie pad will give you a good start. I've used this to really clean up corny kegs.
Might not be as high a polish as you want, but it's an easy start. To go much beyond that, I'd imagine you'd want a polishing wheel or pad on a power drill. Keep going with finer & finer grit paper and then finer polishing compounds.