I've finished varnishing the hull and deck of the kayak I'm building and I used the Halcyon water-based clear gloss varnish. It comes in 1-quart collapsible containers with a screw-on lid, which makes it easier to store than a standard paint can -- you can squeeze it until the liquid is at the spout so there's very little air in the container and little chance for the varnish to form a skin.
I haven't varnished a kayak before, so I don't have a good basis for assessing how it goes on compared to other varnish. I found it easiest to use a foam brush. The instructions say to put on thick coats and they weren't kidding because if the coat is too thin the brush marks won't lay down. It took a while to figure out the amount of varnish to lay down so there weren't brush marks but also no sags or drips. Inclined areas like the sides of the hull were the most difficult in this respect.
I used the method demonstrated by Nick Schade on his videos where he moves down the kayak in short sections, brushing on the varnish, spreading it out, and tipping it back into the wet edge of the previous section. The Halcyon varnish sets up fast enough that this kind of technique minimizing the exposure time of the wet edge was really necessary.
The promotional material for the Halcyon varnish says you can put on five coats in a day, but that's only technically true. The instructions say to put on three coats, then sand before putting on the final coats. The varnish has to dry for 12 hours before you can sand, so the absolute minimum time spans two days. My experience was that most mistakes in the early coats remained visible until sanding, so following the instructions is the way to go.
One big appeal of the Halcyon varnish is that it's water-based, and it was even easier to clean up than I expected. Much easier than even latex paint, so that was super convenient. There wasn't much of smell to it either. Much less than the Interlux non-water-based paint I used on the hull.
Being water based, it was also nice that the varnish could be thinned by just adding water. The instructions say that thinning isn't usually necessary, but I poured varnish out and poured it back into the container often enough in the course of working on the hull and then the deck that I felt like it was getting thicker than ideal. A little bit of water and some squeezing of the container fixed that up.
Given that I'm a novice in using varnish, I'm pleased with the results I got using the Halcyon. I think the flaws are mostly mine and not the varnish. The water-based aspect of the product in terms of odor and cleanup is really good. I'll see if it lasts well.
