Robert Gammon wrote:



Why in the world would you back-prime cement siding? I can see it
with wood, where you nail the siding up against the sheathing, but
cement board needs an air gap to keep condensation from rotting out
the framing. With proper ventilation, painted one side should be fine.


Hardie Panel or Hardie Plank both are heavy paper coated with cement.
When they are in long term contact with moisture, they will fall
apart. Nailed to the frame, there is NO air circulation, so if
moisture gets behind the siding, it will fail.
Failure to prime the backside was a common, almost universal building
practice here in the early 1980s. Almost all of these homes have had to
have their siding replaced as a result of the builders NOT following
mfg's directions with wood siding.

OK, but the OP questions were about Hardie plank and not wood. Hardie
doesn't mention anything about painting the backside.