Winter care of Sunbrella
Zeph, Sunbrella starts out with a tight enough weave and a thread treatment that together make it watertight...but this wanes over time and will eventually start to become permeable after 1-2 years of full-time use. This process is accelerated if you clean the fabric (e.g. with a very mild, diluted soap/water solution and a brush), which of course the fabric eventually needs. Even with lighter colors, folks try to defer washing with any kind of soap for as long as possible. (When we feel we ''must'', we use a heavily diluted Woolite).
The question then becomes what do you use - and how do you apply - a fabric treatment to reestablish repellency. 303 is one common choice, and it usually is sprayed. However, applying your product of choice with a foam brush (on a warm, dry day) will insure the treatment lasts longer. Doing this with the fabric laid out somewhere will help you keep it off the vinyl windows.