A very good question. I've had some luck, but not with re-coating the entire kayak.
My kayak is on its fifth skin. Different skins have had different coatings. But the best luck I've had were 2-part polyurethane over nylon, either Corey's goop or Coelon. I've tried re-coating in the past. once was Coelon over Coelon, where I re-coated the entire bottom of the kayak; once with spray-on Home Armor Flexible Roofing sealer over Coelon, only in the heavy wear areas. My prep work was sadly marginal in both cases. And in both cases these coating peeled within a season. It could have been the lousy prep work.
The kayak had it's last re-skinning, after a major re-build, in 2019, skinned with 10-ounce ballistic nylon and coated with Corey's goop. The kayak gets heavily used and is often launched off of sandy or rocky beaches in small surf. There have been wear along areas of stress, stretch, and abrasion, such as the keel, the chines, and the heavily flexing skin under the coaming. I was getting more leakage than I wanted.
This time, (last Spring) I was going to put another coat of Corey's goop over the entire kayak. That's what Corey suggested. And a few other people thought it would work, after a thorough sanding with 80-grit, and a rubdown with acetone. But only one person had actually done this. and a few years later it was peeling badly. I didn't want that.
So after the thorough sanding and acetone, I used a different approach. I put 3M 4200 on the heavy wear areas using a flexible squeegee. In the hard to get to areas, such as under the coaming, I used Aquaseal. I can't say that it looks very good – that could be my craftsmanship, if you want to call it that – but so far it's worked pretty well.
It's not perfect. First, I put too much 4200 on, I think, It's tight and fairly abrasion resistant. But I added 3-4 pounds to the kayak. And the Aquaseal needs minor renewing every few months. Easy to do and not a big deal. The kayak is pretty tight.
If I had it to do again, I might use Aquaseal all around instead of the 4200. I'm not sure it would handle the abrasion as well, but I'm not sure. And it's a lot lighter and easier to use. But hopefully I won't have to test this for a few more years.
I'd be curious to know what other people have done.
-- Dan Segal