I used to use the traditional mix of 1:1:1 BLO/Beeswax/Paraffin with a little turpentine mixed in for for spreadability until I read of the detrimental effects of BLO on cotton fibers(it thins and makes the fabric brittle over time). I also noticed that once I got to the bottom of my mixture, the BLO seemed to leave bits of rubbery yellow "coagulated" (for lack of a better term) substance that would not melt or dissolve. Over time, I've come up with a much simpler method. Now, I just mix Beeswax and Paraffin (it's used in canning and can be purchased at most grocery stores CHEAPLY) in a ratio of 1:1. I heat it in a pot on the stove until its fully fluid and clear, give it a good mix, and then pour into a little crock pot that is used for keeping sauces warm. They are cheap, small, and keep the wax warm enough to remain fully liquefied. I have a natural hair paint brush and apply the liquid wax in quick even strokes. Once I get a significant area of the canvas coated with the wax, I take a heat gun and (with constant motion) melt the wax into the fibers of the fabric. It is very obvious when it is absorbed because it will darken and become solid color. I work my way up and down the chaps or other canvas item(laying flat on a piece of cardboard - prevents your work area from getting wax covered), working in sections and then apply to the back of the item. If you want an ultra-waxed coating, you can apply to the inside of the item as well. I've done this with canvas shirts, jackets, bags, backpacks, hats, canvas gun cases, and pants. My friend and I usually keep a large folded piece of waxed canvas fabric in hunting jackets or fishing bags for when we need to sit on a damp moss covered rock or log and would prefer not to get wet. I thoroughly enjoy waxed canvas and use it a lot for outdoor activities due to its waterproof and toughness qualities. It also extends the wear of canvas items in my opinion. I do most of my waxing in August due to the very hot temperatures and sun which help to 'prep' the fabric before taking the wax. I also let the items hang in the hot sun for a few days after waxing to obtain full impregnation of the fibers. It may sound like a lot of work, but once you have the gear and technique down, it goes very fast.
Anyways, I will say this again, this technique will make the fabric extremely rigid. If you want armor for busting through greenbrier and multiflora rose thickets, this is the ticket. If you are just looking for a vintage way of waterproofing fabric the more traditional 'oil fabric' approach may be better for you. I will say however, the heavy wax technique mentioned above far supersedes the waterproof effectiveness of the Filson or Barbour wax mixtures. I've used all three in different years and the ability of the heavy wax to shed both water and thorn is superior in my humble opinion. If you have concerns about trying this on a $400 Filson jacket, I suggest you do some tests or experiments on scrap pieces of canvas, or a cheap canvas bag, pants, etc.
Rigid! A thin canvas shooting shirt, turned early season brush buster. As you can see, its holding its form even when held sideways.
