I'd say if you wan't to seal it 100%, head to your local paint shop and pick up some good epoxy primer and a nice outdoors epoxy paint. Oilbase is great but will crack over time and moisture will then get in.
However, we both have to deal with Florida's humidity. Sometimes it is normal, but other times it feels like you can swim in it. When it is really humid outside, the humidity goes up inside somewhat too (not so bad though). I experimented with a dehumidifier, but it would raise the room temperature by about ten degrees.