All my outside props are sealed with Spar Urethane..it gives them a slight gloss but works wonders to protect from rain/humidity. Spar Urethane is available from Home Depot or Lowes...about $30 bucks a gallon but a little goes a long way...I think I used about an 1/8th of a gallon to seal my last eight pumpkins. I've also used polyurethanes, shellacs or stains with sealer. They all work well but Spar Urethane has been the most successful.
Happy Halloween,
Scott