Now, I have tried it on scraps of both nylon and polyester, with great results. It does feel greasy - greasier after soaking for several days in water. But, dried out, the ZAR poly varnish I then applied adhered great - even after a run through the washing machine, with a load of laundry, and then soaking for several more days. And, it's still the most durable coating I've come across.
So, with what seems like good paint adherence, I'm planning on using it for the underwater surfaces of the boat I'm working on now. One thin layer, with a coat of paint/varnish over that. I'll be doing this outdoors, of course, as the fumes are fairly toxic.

I'll report my results . . . eventually.

Application to the inside of the kayak, at your feet, sounds like a good plan. Even if the outer coating scrapes or flakes off, the fabric won't be able to leak there.