The trouble with the fiberglass is usually with the polishes and the waxes that's why cleaning with rubbing alcohol is so important. But I really think the issue is the bottom of the gasket where it can have water pushed up and under the gasket by the air while moving.
I know glass to glass using silicone is great.
Let us know how it works for you. Can't remember the last time it rained here!
__________________
|