Personally, I've never tried it, nor would I unless I were certain I'd never want to camp in a place where it might rain. Just sayin'...
Odds are, the manufacturing process applies waterproofing after the canvas is dyed, which means you'd likely have to remove everything in order to start with a blank canvas.
Yeah... before I'd go through all that, I'd take a look at a product like this =>
Rust-Oleum 248918 Automotive Fabric & Vinyl Spray Paint, 11-Ounce, Gloss Black PS: Check out the Reviews section... lots of comments re: pop-ups.