in Kaboom for a long time to see if it harmed the clearcoat/paint. The results were good: stains came off, paint didn't. I use P21s gel, and if that doesn't work, I use Kaboom, and that always gets it. I still don't trust it 100%, but its worked well for me for a year and a half, and I wash my wheels alot. Once you get them clean again, use wheel wax and stuff comes off easier at subsequent cleanings.

2004 A6 2.7T S-Line
3M clear bra
20% tint rear/30% front
18" RS6 Reps

2006 Honda Odyssey EX-L (Mommy-mobile!)