If Sonax and P21S doesn't work, try clay bar, but I doubt it will work.
I had a set of wheels that had baked brake dust and was about to send them to repaint because nothing would work.
I decided to use oven cleaner right before sending them to painting, because I had nothing to loose.
I sprayed the wheel and let it sit for 2 min and the brake dust just lifted off. Wheels are now like new and no damage was done to the wheels.
Results may vary, but I know that VW wheels did stand up to the oven cleaner. Do not put oven cleaner on the tires though.
Use at your own risk because it may very well damage your wheel finish.
