I've turned a couple of poplar pens, and finished them with wax (Hut PPP). I think that a CA finish would be best, as it is relatively soft and they really didn't have a nice shine. Just my opinion. Also, has anybody ever tried spar urethane for a pen finish? This is supposed to have about the best UV protection, so the wood retains it's color. I have not tried it, but wondered how it would work for stabilizing woods whose colors are light sensitive.