I normaly spray them out really well with a presure washer, get all the big dirt off I then spray a good apc, mixed strongly. (right now im using p21s total auto wash). I let it sit while i suds up the whole car. When i go to rinse the car, i rinse the wheel wells out with a presure washer again, this cleans them really well. From here youo will still see them, and they dont look pretty. They need to be dressed. For this i use somthing cheap, since its the wheel wells. I know thats bad of me, but w.e. Its how i do it. I use either 303 (using up my stock to move onto optimum) or armour all becasue i can get it localy for cheap. It does a good job of dressing them, whitout any ill effect.
This is just how i do it, give it a shot, it should work