Aircraft wash team composition varied as to location.
Kinloss,in the Nimrod era, was a civilian team who washed externally and cleaned internally.St Mawgan was similar. As I recall Lyneham C130 wash team came from those on the Primary servicing teams who were servicemen and from my distant memory Waddington,in the early 70s was too as I remember a Halton mate,who was posted there,lamenting his new duty.
Nimrods had washes carried out for differing reasons. The post flight taxiway wash,the engine compressor wash and the Wash Team wash.
The taxiway,automatic wash using water only, was on request form the captain(not all sorties were in a salt laden atmosphere),as said previously;no proven benefit but probably helped. XV236 was cut up for corrosion inspection and caused a few raised eyebrows by all accounts. A Corrosion Prevention Team was manned by NLS for many years.
The compressor wash (WD40) had two functions,1.To remove salt and 2.To try to prevent sticky engine bleed valves. These were stopped because there was no indicated benefit and also because the application gun nozzle had a nasty habit of working loose during the wash. (one solution being to wirelock it to the lance.)
The Wash Team wash, using power hoses and detergent, as part of Out Of Phase maintenance was carried out every 35 days and also prior to scheduled servicing; to ensure a clean airframe ready for inspection.