I'm prepping my rotor hats for some gloss black POR-15. I had already hit them with Optimum Power Cleaner, a paint safe all purpose cleaner / de-greaser, at full strength a couple times. Then some engine de-greaser. They took it like a champ, still had some brake dust on them that wasn't coming off, even with brush agitation. I then hit them with brake parts cleaner, and, well, you see the results. Lol. Oops. I've been careful to keep the sprays off the calipers.