I use a couple of different products. One is called Maas that works good on most spots and the the other is Mother's Billet polishing compound. It also works pretty good but both of them take some rubbing.
Depending on how bad the spots are, you may have to use wet/dry sandpaper to get the spots out and then go with polishing. I've done a couple of different sets of wheels starting with 600 grit then finishing it off with 1000 grit. After that, I went the polishing route using a 4" die grinder with a buffing wheel.