I was wondering if anyone Ever used a deck stain to waterproof canvas

I never tried deck stain. I did paint some canvas one time to waterproof it for a special project. Of course, the canvas came out very stiff, but that suited my purpose.
I think deck stain might also make the canvas stiff.
Try it on a small scrap piece first.

Yeah, the water proofing portion works, so you can do Thompson's Water Seal or Bear sealant, but often the stain portion as Rancocas mentioned will make the canvas pretty stiff, and anyplace where you fold it the fibers tend to break. I think it's from the drying agent that they add to the stuff.

The simplest answer would be to get some of whatever color you wished, and then do a test strip. If it cures and doesn't make the canvas stiff you're in business. You might also get a small amount of the deck stain and mix it 50/50 with something like Thompsons and do a test strip...and see if you like the color and to find out if it's not as stiff as the straight deck stain.

Be Well

LD

I know I’m late on this, but Townsends’ has got a boatload of videos on various subjects, at least one of which is about painting canvas.

Ive no used Thompsons water seal.What ive always used was interior/exterior latex paint. JA

Thompson's Water Seal used to be the thing to use. However, a number of years ago they changed the formula and I understand that it doesn't work well for fabric any more. I may get some just to try some test strips to see what happens.

I recently made a small lean-to from bed sheets and water proofed it with wax dissolved in turpentine. It seemed to work but I haven't given it a real test.