I wondered the same about the possibility of cracking. But figured they are small so should not expand and contract too much. I painted inside and out so the moisture in the wood is sealed and should remain stable. So far they are like new, but time will tell.
I used to make wind turbines so have used a lot of resins too, mostly epoxy and polyester. I was told way back when that polyurethane was inferior - I don't know why or if that was just in relation to what we used it for.
However after doing a bit of google it looks like polyurethane resins have come a long way in the last 20 years and may have been the better choice.
I think the polyurethane would be a good choice for pine boxes like the op has.
Polyurethanes have certainly improved over the last 25 years and MMAs (which are related to polyesters) are even more flexible both physically and in mixing tolerances.
So thanks for the tip, much appreciated.
Including wood, screws, runners and resin, I can make a ply super for a shade under £8. The cheapness was a real help to get over the expense hump when getting started. If you are short on budget and have the tools, time and skills then I recommend. But be warned ply boxes are indeed heavy.....
If I buy cedar directly from the mill, season and size I can make a super for £6.70 but it takes time and it's so much simpler to pay double and take 25% of the time to put together. - it's also likely to have a better resale value.
I now make the things I can't get in a sale, like underfloor entrance floors, nucs and special boards.
I had 4 supers on a brood+1/2, and lifting the last super to the top of the stack was killa when almost full. Luckily I borrowed a bee suit from work and got a friend to help me put the clearer board on.
So to save my back I got some ceder seconds to build for next year. It will be ply on the bottom and ceder on the top next year.
ps. thanks JBM for giving me the heads up on a sale