Start with soap (not detergent) and stiff brush
Mark:If you link up to the Sunbrella site, they recommend soap (like Ivory Flakes) but not detergent) and a stiff brush.I would suggest that you start with this treatment. If that does not work then you may want to move on to something a little stronger. I would think that a little bleach in the soapy mixture would be fine, but I would check first.Once you get it clean, be sure to give it a good treatment with 303 Sunbrella protectant.