Basically, with Volks, don't use any specific wheel cleaner, just normal car shampoo and water and good old elbow grease.
Now that you have started using a polishing compound on the surface with a polishing wheel, you may have taken the anodisation layer off ( if the buffing cloth turns black after polishing, then yeah, you're on bare metal).
Over time, it will get more and more tedious to keep them polished.
My advice is to get them powdercoated as soon as you can to prevent further surface degradation.