What to use to seal wood in new coop and run?

[Kieh12]

I am looking for something chicken safe that I can use on the run and outside of the coop to make it a little more insect (specifically for ants) resistant as well as prevent too much damage from rain.

I was considering barn paint, but recently was told about linseed oil. However, upon research linseed oil is apparently a great host for mildew. So maybe stain?