can you use a polyurethane finish instead of tung oil? Poly has a UV protectant and will last several years.
Is that like a varnish?
You could I’m sure but as it wears off it would flake, look unsightly and when you need to spruce it up you would have to rub everything back down to the wood. In that respect I feel an oil is better and allows the wood to breathe. You don’t have to coat cedar with anything if you don’t want to though. It has its own natural oils
The only issue I could see with using a urethane type finish is that normally you finish the full piece of wood on all sides. For a hive you are only going to finish the outside, which could cause problems with expansion and contraction due to heat and humidity. But the short answer is that you will only be treating the outside of the hive where they don’t live so even coatings that you would not want on the inside could be used(IE many people paint their hives and you would never paint the inside).