When I did a spot of aeroplane washing last Summer I found that it helped me to understand the aircraft better from an external point of view; all those hours running a long-handled broom over a Cessna's wing on a little ladder is surprisingly effective at increasing your awareness to what you can and can't do to a wing with regards to stress in the air also. May get my hands dirty again this year as well if I can convince the folks to drive me the extra few miles.
Cheers and best of luck, Jack.