Someone here on the list explained last summer how spraying on water
repellents on the canvass is a real no no and will make the canvas
unbreathable and that the canvass is naturally water repellent. 

This will cause moisture to build up in there since waterproofing will make
the water hard to get out through the canvass.

I have sprayed my canvass with water repellent and now it gets clammy up
there and I wished I hadn't. Canvass is naturally waterprooof, according to
last summer's post.