I go to great lengths to protect my year and a half old 225. I clean it regularly and cover it with both a Grady supplied bow cover and a custom made cockpit cover. I've cleaned the vinyl with various vinyl cleaners, ie 3M, West Marine, as well as Vinegar and a product recommended on this forum (cannot recall name, but it's a green bottle purchased from Home Depot) and then treated with 303. The best cleaner seems to be the West Marine Vinyl cleaner, however, numerous small black spots, presumably mildew, still appear on my bolsters and nothing seems to remove it. My seat cushions are fine, but not the bolsters. Thinking moist uncirculated air under the cover must facilitate mildew growth, I leave a small dehumidifier fan running all the time under the cover, but it doesn't seem to help. I had the same problem with the bolsters on my last boat, a Gulfstream 23. Does anyone have any suggestions as to how to get rid of the dark spots on my bolsters?