Just contact a detailing chemical supplier and tell him you need a jug of wheel acid. Carefully do one wheel at a time, while keeping the wheel wet with water to prevent the acid from staining the wheel. Simple process:
-Pressure wash wheel
-Spray wheel acid diluted 50/50 onto wheel surface
-Agitate with tooth brush or wheel brush
-Rinse with water... And then you'll be wishing you started using wheel acid from the begining. I use it everyday and love it.