It's always a good idea to wash the wheel with soap and water first to remove any of the easier to get rid of stuff, then spray properly diluted (4:1) Wheel Brightener liberally onto the wheel and allow it to dwell for 20-30 seconds, then agitate with an appropriate brush and rinse thoroughly. WB is an acidic wheel cleaner with a pH of 5.5 when diluted, so it's fairly potent. Even so, sometimes a second application is needed if the brake dust has actually etched down into the surface. Of course you want to keep it away from bare, high polished aluminum or anodized wheels. Otherwise it's safe on chrome, painted and clear coated wheels. It should be noted, however, that if you have a polished aluminum wheel with a clear coat over it, you want to inspect the wheel for any pitting, cracking or peeling of the clear coat. This often happens when wheel weights are installed/removed, and it is possible for WB to get into and under these problem areas and stain the polished aluminum underneath. But if the wheel is fully sealed you shouldn't have any problems.

I know when I had my factory brake pads on my car and spent a weekend of hammering around Buttonwillow Raceway, even my BACK wheels were black with brake dust. You can then imagine what my front wheels looked like!! I pulled the wheels off the car and used WB to get even the reverse of the wheels looking like new. Granted, the brake dust hadn't sat on there for months, but the back side of the wheels was an almost uniform black.