I think just about any detergent based concoction will work. Add a little bleach to attack the mildew. But here's an additional suggestion: I tried using a spong mop to scrub the underside, but the mop was heavy and a little awkward to use. Furthermore the cleaning solution ran down the mop handle and got all over me. I tried using a swiffer mop and it worked great. I used one of the wet pads but it dried out quickly so I just kept dipping it in my detergent/bleach solution and continued cleaning. The mop is very light weight, it stays in contact with the awning because of its gimbal design, and it didn't hold enough solution to drip.