I used a product called RaggTopp to treat the canvas roof on my old Boxster. The product caused the roof to become water repellant and allowed bird droppings to be rinsed off with a hose. The roof needed to be treated once or twice per year to maintain the water repellancy. It did not discolor the roof and was invisible when applied and dried. I also had luck with another product called Aerospace 303 Protectant, before I discovered the RaggTopp. You need to be careful as some of the products available, such as Porsche water proofer, are difficult to remove from paint or plastic windows and require any area you do not want treated to be taped over. RaggTopp and 303 both come off of paint and windows easily.
