Try Autogeek on line and see if they have a filtering system. I know I have seen one somewhere but its expensive. What I used to do when I had my 2 black PT's was I never washed in direct sunlight, either wash in the shade or after the sun goes down. I waxed/polished the cars once a month and although they still got some spots a damp towel and my detail spray usually took care of them.
Pat