I would recommend using Spar Varnish as per Elkhaven's recommendation. Urethane is extremely tough but along with that is the fact that it is very brittle and not meant to flex. Oars flex, correct??? Use a Spar Varnish which is meant to flex, which will reduce how often you need to refinish the oars...this is why sailors use this product on wood boats and it is the same product that Sawyer uses on their wood oars as well their paddles. Whether the product is Spar Varnish or Spar Urethane, both will have UV inhibitors...that is what the "spar" indicates in most cases. While the Spar Varnish is superior, if you cannot get it the Spar Urethane will do the trick just doesn't last as long and you will see cracking and chipping quicker than with varnish.
If you want to get rid of the "itch" that you get with your Cataracts....send them to Sawyer and they can refinish them such that the fiberglass will not come out. Will not be as glossy as they were, but you will end up with no itch and a killer rope wrap! Just allow yourself some time as they can get pretty backlogged on repairs.
Grand Junction CO