I made some screened bottom boards using pressure-treated plywood. Should I coat them with a varnish or polyurethane (sort of defeats the purpose of using treated wood) so that the bees don't come into contact with the treated wood?
I would definitely coat them with something to keep the bees from being in contact with the chemicals used to treat the wood. The bees will "scrub" the landing board and who knows if they couldn't end up putting some of the chemicals into the hive, comb, honey...
I would not use PT lumber at all for future bottom boards. If it was a good idea I promise you that PT bottom boards would be available from the suppliers.
I would coat them with some kind of varnish/polyurethane, and only use them if you really have to. You could put varnish on the exterior only of 1X pine and have a much more neutral base for the bees, this is the setup I use and it can lost a long time. I feel there are too many metals, bonding agents and what ever else in treated wood that is just not good for the bees or myself. Good luck.
In my experience, nothing sticks too well to fresh PT lumber. If it weathers for a few years then it will hold a top coat but still not very well. My vote would be just not to use it or only use in on the bottom under the screen where the bees don't come in contact.
If you look up pressure treated wood you would find out that there is an insecticide in the chemical make up of the treatment. That is why you use it for decks and outdoor projects and why you have to watch breathing the sawdust among other things. This is just my 2 cents. I would not dream of using it for any part of the hive.:no:
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