Craig, I'd stay away from polyurethane as it's just plastic painted on the wood and will fail in a few years requiring lots of sanding to remove it. Put a stain/sealer on it so it can be refreshed in a few years without it flaking or pealing. Mar-K (now out of business) did a test study on a bunch of finishes on wood they put on the outside of their building back in 2005...if you google it AI will give you the results.
Mike B
