I sanded the third coat of primer with 220, and then 600 grit sand paper. I was able to make a big jump in grit because the primer is fairly soft and the polishing compound that I used is really good. The polishing compound is Meguires Diamondcut 2.0, I think it is the best compound available. You can go from the final sand, to a high gloss finish with 1 compound instead of 3.
