If you speak with the airport owners, they will know what is required (EPA wise) and where you can wash aircraft. Unfortunately as others have said this may cost you.
I for one think the idea certainly has merit. Many aircraft owners are quite fastidious with their aircraft and don't necessarily have time to look after it themselves. Just be sure you are charging enough, I have washed and polished many aircraft and to do it properly takes time, a lot of time!
Good luck with the venture, I applaud you for giving it a go!
