I used to do a bunch of Rev. War re-enacting, and we used wooden barrels, buckets and tubs. These were oak, and uncoated in any way. A few days before we needed to use them we began swelling them. Fill with water, let sit, top off water as required. In short order, i.e. 3-4 hours the leakage would slow to a trickle, after a day or so, there was no leakage at all. After the event, we merely let them dry out, and moved them to storage. I have barrels, and tubs tat are 20 years old, with no rot, and can still be made to hold water. I have never tried this, but I have been told that paint, or other coatings holds the moisture in the wood, and promotes rot.
Ralph M. Reese