One coat fills pretty well. I usually build up three coats. I've done three kayaks in nylon with it, and no troubles. I think it probably weighs a little more then sealing with varnish, but it bet it's about the same as a urethane goop for weight.

It does NOT sand. It's rubbery when dry and toughens up a bit harder as it cures. Once it dries you can scrape, but that usually make things worse.

Oh, and it's UV stable.

I'll admit that I've never striven for a truly smooth coat on a kayak, but yes I think it might. I work in theatre as a designer, and it's become a standard goop for creating texture on theatre and film sets. I have achieved fairly smooth finishes on scenery when trying, but again I have never tried for completely smooth.

It is not limited by any overcoat that I've ever found, or any of the scene painters that I am in contact with. It takes paint, varnish, acrylic sealers etc... One of the advantages in theatre is that it also fireproofs.

I think some tests are in order. I'm building a skin canoe right now. I'll hopefully be skinning it within the month. I'll try and see how smooth I can get it on the nylon by rubbing it out. My fear is that the fiber reinforcement in it may show through. The fibers are fine, but you can see them when brushing it on.

I'm curious about Robs mastic, because it's also fiber reinforced, and he's clearly getting smooth finishes.

I should as note that the MSDS or SDS (whatever they're called now) is about as friendly as you can get in terms of modern toxic products. Much better than Urethane and varnish. That's part of why I tried it on my kayaks.