Walnut or plastic media is the best. It won't harm or warp the metal. Soda (which I just did) is good. We used a high pressure steam cleaner to wash the soda from the pores of the body inside and out.I'm going to have my 71 Nova painted this winter and an trying to decide what type of blasting to have done . I know that soda is out . Now looking at glass , plastic or walnut . Anybody have any of these done?