Weld a stainless steel panel or exhaust tip and that shimmering rainbow discoloration is almost guaranteed to appear. It is not just cosmetic; it is chromium oxidizing and leaving the surface of your metal, which sets the stage for pitting corrosion later. Read on for what is really happening when stainless overheats, the vinegar, cleaner-paste, and mechanical fixes that remove it, and how to keep the rainbow from forming in the first place.