i remember my greddy plenum being a bitch of a thing to install so i never bothered pullin it back off to polish it, but youd have to sand back the coarse finish first then work your way down.
i had whites recomended to me when i wanted to get my radiator done, i ended up doing it myself, i wished id gotten whites to do it (dead arms) i know of another place but i cant think of it off the top of my head atm, lemme get back to ya