If I had a set of finned aluminum valve covers I wanted painted with the aluminum showing through on the fins, I would sand and prep them, spray them with whatever color paint you want. Let then dry well. Then wet sand the fins to remove the paint. First with 400 to get the most of paint off, then finer & finer to get a nice finish. Then if you want to polish the fins hand rub with some rubbing compound. Takes a little patience, but works great.