Waxed canvas is known and loved for being waterproof. The wax coating repels water, making this a popular material that has stood the test of time for constructing military rucksacks & backpacks, sails, tents, and outdoor gear.
Caring properly for waxed canvas will help it remain waterproof. If you preserve the wax coating, you’ll preserve the water resistance. So, for one, go easy on the cleaning. Waxed canvas is meant to show its wear and age, so let it. If you need to clean it, do not dry clean it, and do not put it in the washer or dryer. Use cool water and mild soap (not detergent) if needed. Detergents will break down the wax coating, compromising its ability to repel water.
Another way to ensure your waxed canvas bag remains waterproof is to re-wax it every year or so. You can usually reach out to the manufacturer for this service, or you can give DIY re-waxing a try.
There are many reasons waxed canvas fabric is one of the oldest types of fabric still used today; being waterproof is at the top of the list.