Rims

I do car detailing where i work. We have a Sebring convertable that has something all over the front wheels. They are almost completly covered in it. Ive tried all the cleaners we have and nothing worked. It looks like brake dust but its one there thick. The back wheels have it too but not very bad. They are an aluminum wheel and the only thing i have found that works is steel wool and it scratches the rims a little. I dont know what to try next. Can anyone help me? :confused: