You can use Thompson Water Seal on canvas but why would you want to? Water Seal is a chemical that spreads a thin layer of chemical paraffin wax and fast drying alcohol! Cracking, flaking, and leaking is the result.
The best product to waterproof canvas is the good stuff that contains no chemicals, linseed oil, or turpentine. These are all extremely flammable when melting the waterproofing wax. As well, they do not contain the essential oil to draw the wax deep into the fabric which will cause cracking and flaking.
The Ole Time Woodsman OilSkin Wax Story:
Back in the day, all outdoors folk and the
military relied on a simple yet effective way to
waterproof and preserve their
canvas and cotton outer garments. Using an
old Civil War formula Ole Time Woodsman has recreated an
original, all natural formula that when applied
properly will guarantee that your garment or
hat will be waterproof AND stay that way with
some annual touch ups.
How to apply your Oilskin Wax:
1) Melt Oilskin Wax by placing can in a sauce
pan of water on stove
2) Apply generously with an ordinary paint
brush. Warm with hair dryer and let stand for
24-48 hrs. Several coats may be necessary to
reach your desired waterproofing.
3) Iron garment with a dry iron set on "cotton"
to seal in the wax. (No steam)
Ingredients: Natural Beeswax from Maine, fish oil, beef tallow, and pine resin.