The ceiling in my outdoor lanai in Florida is drywall with papered and
compounded seams. The paper is coming up in places and there are signs this
has been repaird in the past so I want to eliminate the problem. The
ceiling is exposed to outdoor humidity, but completely protected from rain.

I've investigated various solutions, including vinyl beadboard sheets, wood
planking (v-joint or beadboard), and hardipanel. By far the least
expensive, is hardipanel, but the problem is how to handle the seams. An
easy solution is to cover the seams with a batten and try to make it look
decorative, but I'd prefer a completely flat ceiling as I have at present.

Hardie Products recommends caulking seams, but caulk shrinks, can't be
sanded smooth, etc. So, my thought is to leave, say 1/8" (or less) crevices
between panels and using plaster (no joint paper) to fill them, and sand it
smooth. (Hardipanel is very little thermal expansion/contraction.) The
Hardipanel comes in a smooth finish, but it's pre-primed, including the
edges. My questions therefore are primarily as to whether the plaster would
do the job.

1. Will the plaster adhere adequately to the pre-primed hardipanel?

2. If not, if I simply roughed the edges with a coarse file, would that
solve it sufficiently?

3. If you think the plaster will work, and assuming it gets a good coat of
paint, will there be any other long-term issues using plaster?

4. If, instead of intentionally leaving gaps into which plaster could fill
(and hopefully adhere), I butt the seams and just use plaster to fill the
fine remaining crack, does that make the solution better or worse?

5. Do you have any alternative solutions that will give me a ceiling that
has very low long-term maintenance?

Many thanks in advance.