we have just finished our media blasting building. media is the way to go. walnut shells are for engine parts and such. using it on sheet metal will drive oil into the metal. i do not recomend walnut shells. remove your paint with media and then hit the rust spots a low presure with fine sand. any surface rust on flat areas can be polished off with the 3m system. platic is the only thing i know of that wont warp metal.