Hey Greig, So bare with me while I think this thru, You are sort of hitting the proverbial brick wall when you are building up the hive at the brood box only stage. The Flow brood box is exactly the same as a Langstroth brood box. I am starting to think the tung oil might be the issue. I assume that is the only brood box having been tung oiled?? Correct me if I am wrong but I am thinking I have it right.
Till now I was thinking the problem only happened when a Flow Super was added.
The hoop pine and cedar have been used for hive making for a very long time so lets cross that off the suspect list,
I have never heard of the frames having an issue with anyone either personally or on the forum except for people who have bought the Chinese crap foundation that has paraffin wax mixed in with bees wax which is an issue easily spotted by its very lighter color and a lick of your tongue will give you a bad taste, like kerosene.
If your foundation is ok then all we are left with is the tung oil, is that logical.?
I can sense this will get a reply from those that have used tung oil with no issues, but please give me some slack on the noose as I am trying to figure out Greig’s problem.
So what you have is a hive that kills the bees. What about spending some time and a bit of cash to see if we can change the hive to being usable. Give it a light sanding and paint everywhere you treated with tung oil with a primer paint, another light sanding and hit it again with primer. Leave it 2 weeks then paint it with a reputable white exterior paint with 2 coats and leave it for 2 weeks. Paint the top and bottom edges as well as this is where any rain water accumulates and sits, it is where the timber degrades first.
Put the hive to use and we might have a fix for the issue, A few hours labor and a few bucks of paint and I suspect the problem will be gone, and I’m looking forward to a positive update from you.
While Jeff prefers the convention Langstroth hives and is set up for them he has a great knowledge in bee keeping and is no goose about bee keeping with many years at it.
The way you do your splits is good, I am thinking tung oil fumes.
Regards