Applying varnish in cold weather

I need to apply some System Three Spar Urethane Varnish to my stitch and glue boat, ideally before spring. The high temps lately are about 30 degrees. I don’t see anything on the can, or on the technical data sheet that I downloaded, about ideal temps for application. Is it okay to apply urethane varnish in cold weather?

My Captain’s varnish…
…says not under 40 degrees…

Think I would wait…

not recomended
With slow drying times varnish can develop a milky white blush. Best to either heat your workplace or wait until spring.

Can you heat your workspace?
I work on my wood boat in a room that is normally unheated. I put two portable oil-filled radiators in there. They work great. The room also has enough big windows that even on a 20-deg day that’s SUNNY I can shut both of them off in the “heat of the day” and it stays above 65 deg.

Even if the varnish cures at cold temps, it will take longer, which means more likelihood of dust and other crud settling in the sticky finish. I wouldn’t do it. You’re likely to have to sand off the flawed finish and re-do it.

I emailed System Three and they replied (promptly) that it is not to be applied below 50 F. Curiously, this information is not on the can or the Technical Data Sheet.

My workshop is my garage, with no insulation. I could probably raise the temp a bit with some small heaters I have, but I think I’ll wait for a warmer week.

No rush
an epoxy S&G will be fine to use for a while with no varnish. The Pygmy building book says just that. Don’t wait years, but a few months of use should not cause problems. My wife’s Arctic Tern 14 is fine after a summer’s use in the raw. It’s kept in the garage when it’s not on the water, though. Wait 'til it’s warm.


Yep, I paddled it
without varnish last summer (finished it in June.) I kept it in the garage and out of the sun as much as possible, and it still looks great. I kept putting off varnishing it in the Fall, because mild weather kept providing more opportunities to paddle. I also need to install hatches, bulkheads, thigh braces, and a better seat. I thought it made more sense to varnish it BEFORE cutting the hatches and creating more edges for drips, etc. But I’m bored now, so I think I’ll do the other stuff while it is cold, and varnish later. I managed to get the boat down into my warm basement so I can work in comfort. But I’d probably kill the whole family with the fumes if I varnished it down there.

Do the hatches first…
I you varnish first you will end up having to remove some varnish to epoxy and will get epoxy on top of some varnish. A lot more work and needless mess and you will end up having to revarnish anyway…

do all the other stuff
since you’re putting down four-five coats of varnish you want to put it on finished epoxy and not have to cut through all those nice coats of varnish to re-apply epoxy, sand, recoat, sand, varnish, etc.

The edges of the hatches require some worthwhile finishing which usually gets short shrift with thin coatings of sanded epoxy when it should be thicker than anywhere given that dinging the edges of the 4mm ply cauases water to be sucked up into the edge grain and causing waterstaining. That’s something you can do in the house, bring the hatches in the house, get them nice and warm and apply epoxy on the freshly sanded edges, then take it in the garage. That should help draw the epoxy into the edges. I thicked the epoxy a bit with cabosil so it wouldn’t run off easily. Just enough to make it less runny not to color it.

Good advice. I’ll follow it.