This is an old thread, but I wanted to add to the discussion. If no one responds, I'll consider starting a new topic.
I read with interest the tutorial posted by Forest Turtle. The process he described for waterproofing canvas may be a more effective technique than the one I am going to post. I copied it from the original bush-lore book, Woodcraft, written by Nessmuk (pen-name of George Washington Sears) back in the early 1880's, published in 1884, and still in print. It's WAY old-fashioned...it's 133 years old, and he was no spring chicken when he wrote it, so likely it was old-fashioned when it was published. It's a bit hard to read, with many dated terms -- some so obscure that I never figured out what they meant. But that old man really knew his stuff. He lived it for so many years. My dad was given the book by HIS dad when he was a kid, and he treasured it...gave it to me 63 years ago when I was 10. I loved it then, and still do. My copy is dog-eared, but still readable. I hope my grand-kids will take the time to read it. If they do, they'll treasure it, too.
Nessmuk described a simple process for waterproofing any cotton material, from muslim to denim to canvas. i'm pretty sure it would work on other natural-fiber material, such as linen (flax). I don't believe it will work on man-made fibers, but we never tried that. In 1957 or 58, my dad used it to waterproof a tent and two light cotton rainbreaker jackets for us, and it worked well. The fabric stayed supple. It seemed to last a long time...a couple of years as I recall, maybe more.
Anyway, here's Nessmuk's recipe:
"The cloth does...need a little waterproofing: for which the following receipt [recipe] will answer very well, and add little or nothing to the weight: To 10 quarts of water add 10 ounces of lime, and 4 ounces of alum; let it stand until clear; fold the cloth...and put it in another vessel, pour the solution on it, let it soak for 12 hours; then rinse in luke-warm rain water, stretch and dry in the sun...". Woodcraft, p. 31
There are a lot of different chemical compounds known as alum, but Dad figured the most common and available in Nessmuk's day was potash alum (aluminum potassium sulfate), so that's what he used. There are also three or more types of lime...but since Nessmuk didn't specify (and because he was a woodsman and trekker, not a chemist), Dad figured he'd have used common powdered agricultural lime (mostly calcium carbonate). Dad made a triple-batch, wound up with almost eight gallons...even a small tent takes a lot of liquid to immerse it. Following Nessmuk's "receipt", he added the lime and alum to the water and let it stand...but the lime didn't dissolve. After stirring and waiting, the liquid did settle and clear, with some lime on the bottom. Absent any other direction, Dad just stirred it up and poured all of it over the canvas items in a plastic kiddie-pool. He let them soak for more time than directed (about 24 hours, I think), then rinsed all the items in clean, SOFT, well water (not having any rainwater handy). After he'd rinsed them enough to remove any residual lime, we dried everything in the yard (we erected the tent, hung the jackets).
And like I said, it worked. It may not have worked as well as Scotchguard, but the modern water-repellents weren't available in 1957. And now 3M says it will stop producing Scotchguard because it's not environmentally friendly. So maybe the old ways will stage a come-back -- old Nessmuk rides again!
