Nylon is a hydrophilic fiber, it loves water, and has, like, 300% stretch. You have to sew it on soaking wet, and tighter than a tick. It will still loosen after a couple hours in the water or a week of rainstorms. To some degree, Corey's goop helps "lock" the fibers if you can get it good and tight. Since it shrinks when dry, nylon can crush weak frames. I've never known heat shrinking, ironing, to do a lick of good on nylon.

Polyester is not quite as strong, and it's more difficult to sew, especially in the heavier weights, but it has very little if any stretch, and doesn't loosen in water. On the other hand, you can coat it with pretty much whatever you please.