I used urethane on wood planters that have been outside for 2 yrs now. I did the inside and outside. They're not barrels, but made out of wood scraps & plywood scraps. You would still want to have drain holes (unless not exposed to rain.) Lining drain hole with window screen would help prevent clogging. If you make the holes before you do the urethane, the sides/edges of the holes should get covered with the sealant too.
Covering with a tarp or moving barrel to a covered or enclosed spot for winter would help keep it from aging so quickly also, if possible.
