Here are several ideas.

My dad is at Ocala with the same drywall soffit/ceilings on the
covered patio and front porch. Yes, the tape came undone. I
bought a bag of thermal setting compound and redid the joints.
That was 10 years ago, still holding fine.

If you are going ahead with the Hardi, I would suggest H-mold or
trim at the joints. I don't think you will ever get an even
application of anything to hold on the joints. I have used the
sheet Hardi material, though I usually use one of the textured
panels. The caulked joints show. It is a great material with
trim on the joints, perhaps one of the artificial trims like Azec.
http://www.azek.com/

You might also price any of the plastic siding soffits.
http://www.royalbuildingproducts.com/products/siding/soffit/index.html


--
~~~~~~~~~~~~~~~~~~~~
DanG
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a sophisticated and elegant way to say I LOVE YOU!
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"BobAtVandy" wrote in message
...
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.