There are lot of accessories available specially for Porsche. You can buy wheel cleaning kit as well as Car Care kit which are easily available on car accessories showroom. I know one particular showroom in Times Square mall. Its called Yellow hat. You can get cleaning kit for Porsche that will help in maintaining a long life of the alloy wheels. Also, make sure that when you are driving your car on a construction road or sand area, your wheels are properly balanced and your speed is under control.