A stack of recently painted poly nucs got blown over in high winds during the winter. Inevitably, when they land on the edges of the boxes, the thinner bits break away. These are easy to fix … Gorilla glue is your friend. A smear across one face of the break and a bit of water spread across the other, coupled with some clamps or string to hold everything mated together properly, will form a really strong bond. The glue expands to fill the spaces, oozing out of the joint. Pare back the dried glue 24 hours later and everything will be fine.
Gorilla glue and a little additional ingenuity can also successfully fix boxes that are more dramatically damaged. I dropped a full poly super from shoulder height when harvesting OSR honey in 2014. The result was not pretty, though I did manage to recover most of the frames for extraction. Two of the corners of the poly super (Swienty) were badly broken, with the interlocking lugs snapped off cleanly. A combination of Gorilla glue, dowels and sash clamps fixed it for use during the main flow later in the season.
More extensive gaps such as woodpecker damage can be fixed using exterior wood filler. Fortunately, I’ve not yet had to do this. However, while butchering Paynes 6 frame poly nucs to create 8 frame boxes I filled very large gaps with a combination of Screwfix MegaGrip glue and offcuts of polystyrene. These appear to have lasted well. The glue seemed to take a very long time to set, but it’s rock solid and can be sanded back flush to provide a smooth and easily cleaned face to the repair.