I'm building a Chester Yawl up here in Northern WI. Like most folks, I do not have a heated shop. I really wanted to get the outer hull painted the the Interlux one part urethane prior to winter really closing in.
I made a first coat attempt with temps in the 40's. I had 5, 250W heat lamps suspended over the length of the hull with a 1000W, portable electric baseboard heater under the hull. So the hull was warm enough and the air immediately adjacent to the hull was warm enough. I kept the paint in the house so it was warm enough. No worries, right? Wrong. The paint cooled off so fast in the roller tray that the first coat went on horribly. Hard to keep the roller from loading up with too much paint and it just didn't flow right. If I would have had the specified thinner, maybe that would have helped. But I didn't. Ended up sanding most of that coat off. Might have done a bit of swearing whilst sanding..
I had a simple idea for the 2nd attempt. I bought a number of those disposable hand/pocket/body/toe warmers. The kind that have an exothermic reaction upon contact with air. No flame, nice gentle heat that lasts for hours. I taped several of these to the bottom of the roller pan, one to the bottom of the paint can, and stuffed a small one into the roller itself. These had the effect of keeping the paint warm throughout the 2nd coat effort. This, combined with the under and over hull heat led to a beautiful 2nd coat - despite temps in the 40's.
Going to try this with some varnishing next...