I varnished my first kayak and used AwlBrite 2-part poly clearcoat by Sikkens on my second. I've done pretty gentle launchings and landing, but have made some scratches in the hull. To answer one of your questions, although the surface is harder with 2-part, it's not a substitue for a layer of cloth. I have found that scratches in the 2-part aren't as deep as scratches in varnish, but from an appearance standpoint they look the same. Neither varnish nor 2-part poly hold up to rocks well. Both end up with off-white colored scratches.

I think where the 2-part poly is advantageous is on the deck. You'll avoid some of the scratches you get from sliding things under bungees, stowing paddles, strapping the kayak on your car, etc.

Having done both, I think I'd do varnish next time. I don't think the medical risks of working with 2-part polys are worth it, unless you've got a nice pressurized air suit setup or something.

Dean