I have a 2009 Aqua Patio which I leave covered when not in use. However, each time I uncover it, the tops of the vinyl upholstery, especially in the front of the boat have a dark purple stain on them. As the boat is exposed to sunlight, the color fades to a light pink/brown but never totally disappears. Thinking the cause might be the factory cover, I began placing terry cloth bath towels on top of the upholstery to keep it from touching the cover but that has made no difference. Can you help?