Once we'd gotten the board and batten up, we had to paint it.....

We chose to spray paint our board and batten exterior walls with an exterior water-based latex paint. We purchased a Wagner power sprayer (non-compressed air version) that holds around a gallon of paint. While that's a little less than we'd have liked, it made it very portable, so we just got used to filling it up a lot. It made quick work of the job as well. It took about 2 days to complete and had we not had to climb and move a ladder around constantly, it would have been less. Cleanup was a breeze too.