Luke

what huck said. Move with the shape of the panel but before you attack it with a mitt get all the dirt you can off with a pressure washer and rinse your mitt off after every panel.
Dont press hard, just use the weight of the mitt for pressure and kinda drag i around.
Failing that, touchless technique. (i have black and it will never see a mitt due to it will swirl)
Soak it in a pre wash foam first to help everything moving off then blast it with a pressure washer.