FWIW, here's a link to the manufacturer's recommended cleaning and waterproofing method:
http://www.sunbrella.com/cleaning/ma...ops-covers.php.
I'm about to tackle my dark green covers, and I'll stick pretty close to these recommendations. The 303 waterproofing is available from places like West Marine, but it's pricey. You can save a few bucks buying it on Amazon.