id stick to windows if i were you.pure water is more aggressive esp on certain types of paintwork and i dont think its really great for painted wooden windows either!
google dionised water and read what it says!you could easly damage someones car with a wfp and what about the wheels?do you use the same brush to then clean your windows?
i clean my van with wfp but use tap water instead of pure water and use a different brush!!
im using a bucket and sponge though at the moment due to hosepipe ban!