A. Rust is simply the oxidation of untreated metal surfaces when they come in contact with the elements. Moisture is the main cause of rust. And since dirt attracts and traps moisture, a dirty car is the instigator of almost all car rust, especially in those hidden areas behind chrome and trim. Only a professional carwash operator has the equipment and know-how to effectively reach all those hard-to-reach places and remove corrosion-producing grime before damage is done.