I'm told and the instructions seem to indicate that you can use the dry wash products on the dirtiest of surfaces. They say that the "stuff" they use encapsulates the dirt and protects the surface. That may be 100% accurate but I could never see myself spraying heavy dirt and rubbing it around on my painted surface!! I use Wash Wax All frequently but if the coach is "Alaska" dirty, I wash with good old soap and water first. On our recent trip to Alaska I think we washed with water about 4 times including once at a Blue Beacon in Arizona or NM on the way to the Balloon festival. Then Wash Wax All to clean up the streaks and renew the shine before the festival. Had to look good while parked next to all these "other" coaches!