Reply by RonInOhio

Marine spar varnish is good stuff. But its expensive. I would do as some have suggested and if possible cover the chair or bring it inside for the winter and harsh weather. Doing this will prolong the time between touch-up finishing.

Another tip I saw on an Adirondack build was the author used epoxy around the lower couple of inches and bottom of the chairs legs/feet. Those areas tend to sink into the ground and are at the most risk for rot and splitting.