There are two kinds of wash too.
A "cosmetic wash" involves cleaning the airframe to make it look nice and a "Technical wash" that involves "jet washing" the gear, flaps and various bays. This is done before a maintenance input to enable these areas to be inspected properly. As part of the input all corrosion protection and grease removed by the jet washing will be replaced.
In this enviromentaly friendly age there have to be special wash bays so that the "Dirty water" can be collected and treated before it is poured down the drain.
Can't think of anything more!!!
Rgds Dr.I.