FYI, that black stuff on your rags isn't oxidation. It's actual aluminum that's coming off with the abrasive action of the polish. You'll never get to the point where it doesn't come off.

For a protectant, I'd think some of the tougher clear coats would be just the thing. That's what they do to the high polish wheels for cars to keep them bright. If you want to pursue it I'd check with a body shop and ask if they'd hit your spinner with what's left in the sprayer after doing a urethane clear coat on a car.