You can make it as involved and complex as you want, but I would just use a real good Exterior Polyurethane. I'd sand it down to open the grain a bit, then thin the first coat and put it on heavy and let it soak in for a while. Wipe off excess, and after it dries over night, paint it with poly a few times letting each coat dry for a day in between. It makes it dry better in the long run. I have done this to benches years ago, and they only get screwed up if I do something dumb, but normally they hold up well to use as a gun bench or work bench.
In general urethanes dry harder if coats are applied rather quickly rather than long drying stretches between coats. The polymers link between coats much better, rather than just within the coats. Can directions will give a strong clue about that and there are exceptions.
