I've seen a few other instances where someone sanded through their epoxy primer coat while blocking and found rust UNDER the epoxy where it had been previously blasted clean. In all cases the media that had been used was sand. Most sands contain caustic salts that, when used for a blasting media, are being pounded into your sheet metal. It may be about the cheapest media out there, but it is not ideal.. I think you'll find the instant money savings results in more effort to clean that salt residue. I prefer an inert media, such as Garnet or Coal slag. They have less chance of these adverse effects, and are well worth the extra money. Especially now that the paint substrate adhesion is questionable..
