Avon developed their process called calendering , where the fabric was sprayed onto both sides with hypalon rubber ; and then run through rollers , so as to force the rubber into and thru their fabric ;creating a composite. Other manufactures would try to use a heavier / tighter weave fabric and just coat both sides ; thinking that it would be stronger. These were prone to having bubbles appear as the boat aged. Also variations of polyester vs nylon threads were also incorporated by other manufactures . stretch vs. stiffness . I don't think that any tried a rip stop material that was popular for a while in outdoor fabrics. I think that Avon style boats are still available through Zodiac , under the name of Mil-Pro , If you are willing to pay and to wait.