Joint compound or plaster

I have a buildt-in bookcase with a 1/2" ply back that [nearly] meets up with a wall that is slanted in at the top at 45 degrees. The back of the case and the wall are painted with the same flat wall paint. I want to put a cove at the junction of the plywood and drywall, probably with a 2-3" radius. That means that the material I build the cove out of will be up to 3/8" thick. I have both plaster of paris and drywall joint compound. Which should I use and why?
PS: A molding strip is out--I want to camouflage this intersection, not highlight it.
PPS: I guess that Lew is going to suggest epoxy with microbubbles. How hard would that be versus plaster or mud, and what would be the advantages?
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Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

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Neither a flexible paintable caulk, it will not crack as joint compound or plaster will.
Mark http://home.mchsi.com/~xphome /
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How would I get a good surface on that? This "fairing" will be a little over 2" wide if I go with a 3" radius. That's a LOT of caulk.
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Alex -- Replace "nospam" with "mail" to reply by email. Checked infrequently.

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A wood cove moulding, caulk to fill a small gap at the wall? If you want a plaster cove attach it to the wall but not the bookcase, caulk the joint at the bookcase. How you make plaster cove, I've seen it done but am not going to attempt it.
Mark
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alexy wrote:

Use both. Do the first coat with the plaster of paris (which is basically what setting type joint compound is)over fiberglass tape and then use the premixed joint compound for the finish coat(s).
Why? Plaster will shrink less and dry harder. Joint compound shrinks more, but sands easier.
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Plaster or joint compound will crack. Bookcases have a tendency to move around a little. I'd use a wooden cove painted the same color with paintable caulk. The caulk can flex a small amount which is what you want.
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