I used to work for the Haartz Corporation in Acton MA which made several marine fabric constructions. I was focused on developing automotive interior trim products, but had some exposure to all products. Sunbrella is a solution dyed acrylic fabric. Unfortunately since it is only a fabric it will never be truly 100% waterproof. Haartz makes a coated product called Seamark with combines the fabric with .007” of vinyl making it 100% waterproof. I shot an e-mail over to the product development manager for this area of the business and he recommended a product called Raggtop Fabric Protectant. You can find it at
www.wolfsteins.com. He reiterrated the fact that you should not use silicone.