It's a lovely place that you have there. Do you think there is a definite reason to use something as strong coating as spar urethane? I finished the interior walls of a cabin once with a natural Perilla oil, thinned a bit with mineral spirits. Two coats. A few different types of oil that would work come to mind. It gave a shine and was considerably softer looking than urethane would be, and you can actually still feel the wood. I do generally hate urethane as a wood finish, so am biased. Oil seems adequate for interior, some offering better moisture protection than others, will never flake off or turn yellow, and can be done again at any time without removing the existing finish. I just wonder why such heavy duty is needed inside? Oils with dryers in them will also work on metal.