Best way to prep rallye wheels for paint?

Metal is metal, so remove the valve stems. Media blast them and then hit them with some self etching primer, sand and then use an air chuck to dust off and hit them with paint. Or just media blast and then paint.
It should be relatively cheap to have these blasted at a shop, if not paint stripper will work, but you need to neutralize the chemicals and let is set a while before you try to paint them.