Liquid silicone. Any decent camping store, and even mass retailers like Canadian Tire and (I think) Wally World carry liquid silicone in small or large paint cans somewhere in the camping aisle. Give any fabric a coat or two and it's pretty tight. Repeat with a light coat for each subsequent season. (Avoid the spray-on equivalents, since you get 1/3 the mileage for the same price.)

But: (after dissing spray-on products), I can say that I've also had good luck with ScotchGuard, the 3M product. My experience is with the original formula, no longer in production; I used it on clothing, boots, and tents with good results. I can't give info on the current formulae, though it's from a solid company.

Hope this helps,
-Doug