Originally Posted by wanderingfarandnear
I'm not sure how heavy it makes fabric or if it degrades fabric over a long time, but when I was in high school, I did a science fair project of using different waterproofers on ripstop to see which would work the best. Thompson's watersealer for decks actually was by far the best. It says it's not made for fabric so it might degrade it, but short term it worked great and got me third place.
My father used to apply it to his felt hat when he worked as a cowboy. It made the hat waterproof but also very stiff; so stiff that he used it as a hard hat of sorts, for protection from branches and such when chasing stray cattle out of the bush. I've not had much experience with it but I wonder how much elasticity it retains (i.e., how will it behave when the fabric stretches?).