Something you may want to try is Thompson's water sealer. It comes in a spray can and is easy to use on a frame. I put that on a boat I am building now, so can't say for sure how it will work, but based on other uses, including my home-made kayak car rack, it stands up to moisture very well. I have no idea what type of chemical is in it. It dries very quickly and one coat is enough. With 2 it gets kind of sticky. Water beads up like a heavy varnish, but the rough wood texture is still preserved.