HI:my method is to tell them I wont be able to do a perfect clean 1st time cus a/ if its uvpc the black rubber gathers dirt and rubberizes and if I clean it heavy, experience shows it ends up on the whites and stains the frames,tell the customer have they ever cleaned a car after washing it drying the windows end up with black on the rag,well now you know where it comes from,youve got it luv the rubber around the window.
b/if its old wooden windows tell them you may see lines around the edge because the paint has emulissified [spelling]
and we have trouble with white marks around the window frames.
tell them they will be perfect 2nd clean
never failed for me and gets them use to having a window cleaner come every 4 weeks