Tassadar, when you return the car,or when they come to pick up when you are done detailing, suggest to the owners that before they do a burnout at the track(or wherever) that they should spray some Pledge (furniture polish)on their rear quarters. It works for me--it makes removal a lot easier and doesnt harm the paint finish. The rubber doesnt really adhere to the pledge and will wipe off easily with some tar remover without really rubbing.
bill g