This is like the Sonax Full Effect Wheel Cleaner, Car Pro Iron-X and some others that turn color as they attack the ferrous particles. Iron-X can even be sprayed over the entire paint surface of a car that has heavy bonded contaminants and makes claying of those surfaces much faster, if not completely unnecessary. They all smell absolutely horrid, no doubt about it, but they work well (if a bit slowly) and are safe on even delicate highly polished billet aluminum. This particular product, at $20 for a 32 oz bottle, is actually a pretty decent deal.