Don't wash my RC's. They operate mostly in an on-road and dry grass environment.
Back in the day running 1/8 nitro bug, I'd use compressed air for the initial cleaning. When the mud dried would chip the big stuff off with a wooden stick or rod. Small stuff in nooks and crannies got got with a bristle paint brush. Soap and water and stiff bristle nylon brush cleaned the dis-mounted tires. Keeping a finger over any vent hole(s) while doing that.
Only time anything on the chassis got washed was on tear-down. Hope this answers the mail. Good luck. Cheers. 'AC'