Options....
Vlad, I had one of Bruce's covers, when it lost it's water repellency?....I took it home and sprayed with Scotchguard. Supposedly this teflon or silicon type of surface treatment doesn't totally stop the breathing of the fabric, but it was really a last resort to extend the life of the fabric.
Larry may be entirely correct, but I wasn't too worried about the mildew part, as long as the wind blows, the thing dries out. The worst part was those spring and fall days where it melted the snow or frost, then refroze overnite.
It was totally GLUED on one morning....no flying that day!
so.....
after 10 years and a few washings & restitching, it was toast, and had to be replaced.