Here is what I have done:
At least two coats, three is better. I don't use varnish. I use an outdoor Marine Spar urethane. Minwax Helmsman to be specific. Buy a gallon can and roll it. Don't use the spray cans. Although the spray is really easy to apply, it is mostly aerosol and not much in protective coverage. Spray cans are very costly
For a clear finish, I am now applying a clear natural stain ( Like Watco brand) to the bare wood before the urethane. It seems to hold up better in the wet weather and the oils in the stain conditions the wood. Let it soak in overnight.
Brush urethane into the handle area well. Don't worry about runs, you'll be going over it again almost immediately with a roller. I use a 1/2" nap weanie roller and put it in a big ziplock plastic bag in between coats.
Roll the rest on the box-don't brush it. Load your roller with urethane and start at the end grain box joint ends, roll slowly from the bottom up, pushing the liquid urethane in a pool above your roller. You'll see the urethane soak into the ends where the wood is the most porous. Then roll the remainder of that side of the box. Light coat, side to side-top to bottom, no matter. Just be sure to work it into all the wood. Move on the next side of the box and repeat..etc.
After a few minutes, go over the box with the same roller with no urethane on it to roll out any runs. Now watch out for runs around the handle area until it sets slightly.
Let it dry several hours before repeating with another coat. If temps are cool let it dry overnight between coats
If you lightly sand your box before finishing and use
gloss spar urethane, your finished box will have a smooth as glass finish.
If you have some rough sawn wood and apply it on a horizontal surface, you can really slop on the urethane, LOL. Nice thick coats! Thin coat are necessary on smooth wood.
The trick is to avoid the runs.
Here is an example of the finishes. The mating nuc fence hanging mini double box's are burned with propane torch, then have one coat of Walnut stain applied to condition the wood. The 8 frame deeps have a Watco dark walnut stain applied first. Both have three coats of Spar urethane. Yo can see how glassy the smooth wood it. And these were not sanded.(They are all totally dry.)
I have a few turntables and stack my box's four high, then apply my finishes. This allows me to stand in one place and make good use of table space. I just spin the stack and apply.
If a few years later, you find your box is looking weak, you can easily take a propane torch and burn off the old finish This leaves the box prepped to paint or accept a new urethane finish.
I will say this, just like beekeeping, your hive finishes need to be climate specific. I hear in a hot climate like Texas, urethane will just crack. My climate in the Pacific Northwest is very wet and mild so it is a good method for me.
You area absolutly right, winter time is great for building and finishing! I've been building mating nuc box's and mini frames in the barn while keeping my eye on two very large girlfriends
Also made use of some of the plywood scraps left over from hive building..new nest box's for the rex rabbits.
They hang right over the compost piles
There was a thread here about meat rabbits I've been meaning to address. Just need a rainy day to take time on the computer to do it.
Hope this helps!