Sand the rims until all of the casting ripple/pebbly surface is completely smooth, and then go up through the grades of sandpaper until you have a proper base to use metal polishing compound. The better you level everything out, the better results you'll get in the end. It's very simple actually, just very labor intensive. This is just like color sanding paint, the premise is to knock everything down the the same level, then buff it up to a high shine.