My dad works in Exterior Appearance for American. Here in San Diego, we are one of the few cities that has a washrack, which is the area where the planes are washed. (For those familiar with SAN, the washrach is just west of the commuter terminal). The process used is much the same as at a carwash... The plane is soaked with water, then sprayed with soap. The plane is scrubbed with brushes resembling mops on long poles, and then it is rinsed back off. American washes something like 8 planes per night in SAN, and a commensurate number at their other washracks (I don't know where te others are). After it's cleaned, the plane is towed back to the terminal. This is done at night (my dad's shift is 2030-0530) when the planes are just sitting at the terminal anyway.