I'm a big fan of 303 Aerospace Protectant, but I was really disappointed in their fabric guard. I bought a gallon of 303 Fabric Guard to treat my cover. I screwed up and did not finish the whole cover before running out. I was not able to find the same product locally that day, so I finished the job with a quart of Starbrite. Every time it rained, you could clearly see the difference -- the water beaded up on the Starbrite section. Not on the 303 parts.
Great info, I have used the 303 product for a couple years now. It seems to hold up well for a couple months then deteriorates quickly. It might be worth trying the Starbright product to compare results.