It will be difficult to get a good tape line because masking tape does not like to stick to freshly buffed surfaces. Also, my understanding is you want to polish the edges of the spokes, making it very difficult to get a good tape line on the spokes if you buff first. If you paint first, you can file/ sand a very nice straight edge on the spokes and then buff. I would definitely sand the entire wheel first, mask off the surfaces to be buffed, paint the wheels, let the paint cure a few days, then mask off the painted surfaces, then final sand and buff the polished surfaces. This is how the pros do it. having said that, there is definitely more than one way to skin a cat.