It's an old barn, (I mean _real_ old, older than the US). Pole and beam
construction with vertical plank sheathing. There's a gap between the
planks and most of them have knotholes. No historical district,
register of historical places, or other restrictions except the normal
building codes, under which the barn is grandfathered.

The desired result is to more or less maintain the appearance of the
barn inside and out but have it weather- and critter-tight. It's going
to be used for a garage, workshop, and storage, not living space.

Seems to me that the planks have to come off, sheathing has to go up,
and then the planks have to go back, furred out so that rain coming
through those gaps and knotholes doesn't get trapped. Anyone have
another idea on this?

Also, what to use for sheathing? Zip system? Exterior ply and paint it
black? Extira? Pressure treated ply? Remember that it's going to be
fairly well protected from sun but it's still going to be getting rain
and snow through those gaps and knotholes.

Or should I, when I pull the planks, plug the knotholes from behind
leaving just enough edge for show, mill the edges for battens to fill
the gaps, and then just put zip system or whatever behind it?

Not gonna start on it until Spring regardless but want to be ready to go
when the warm weather returns.