You could use NeverWet, which is two-part spray that makes things, including canvas, waterproof. It's around $10.00 at hardware stores and Wal-Mart.
You could use a clear polyurethane spray in a can like spray paint. I was so excited when Neverwet became available, but I read a lot of stuff about it not being durable that I've never used it. If you do, I'd love to know the longterm results. I have no personal experience with it
Whatever you use make sure it's mold and mildew proof. There are some sealer's used for marine applications that may work. They come in sprays or can be rolled on like paint. I would test to make sure whatever you use doesn't affect the painting itself.
I applied two coats of polyurethane followed by two coats of Golden UV varnish on my mural... be sure to seal front ,sides and back of your canvas to protect the cloth.
golden UV varnish? Is golden a brand name?
Marine finish gurrentied to keep dry. I think its called Spar.
Outdoor Mod Podge would probably work and is not expensive. You could also use a clear spray shelac from Home Depot, Michaels, etc. The other possibility is multiple coats of clear floor finish on the canvas like when you are making a floor cloth.