One of my winter jobs is to repair any damage to the Sunbrella items we have. The IP sail cover is the first ever we have had that was lined, but only an area where the main body of the sail is. In this process I also use the 303 brand Sunbrella/canvas cleaner in the spring followed by the 303 brand of waterproofer. I guess I have a certain amount of skepticism about how a physical thing such as cloth can stop blocking UV? How is that possible? Do the individual fibers shrink and leave space or does the UV pass through the cloth itself?? Any thoughts Bruce?