How to clean Sunbrella fabrics?
There are quite a few good canvas/sail cleaners on the market. (Or, if you prefer, the Lysol laundry product works great - just be sure to test for color fastness. And you will have to scrub with a stiff brush.) And, once you get it good and clean, you may want to think about a good waterproofer if mildew is a problem. What you would do is clean your cover and let it dry TOTALLY, then coat it with the waterproofing. And PLEASE don''t do as a customer of mine recently said he was going to do it - stuff it in a bucket of waterproofing. You would only get streaks, as in a tie dye formation. Lay your cover out on the ground and use a sponge to spread the waterproofing over it, covering all the area.
Hope this helps,