Keep in mind that a layer of oxidation isn't completely a bad thing. That layer (as ugly as it ay look) is keeping your rim from further oxidizing. It's that further oxidation that causes the real problems, like metal pitting.
I use Mothers to keep them polished but when they get like that sometimes it's easier to remove the oxidation with "Neverdull". You can get it at hardware stores...it works great even just to use to polish them.