Honest answer from me .... no idea.
Would I tape the joint?, Yes and would not even think twice about it.
I wish more people knew about plaster, it seems like a lost art sometimes.
I am still new to it and learning more all the time. I would like to read more about it.
Last week I was working on a fireplace insert, it had a gas fireplace already, Was a new smaller unit going in and whole wall needed work.
The bonco was made of plywood covered with cement board and 12" pink tiles.
Took about 1 hour to remove the tiles with a hammer and chisel, this left a crappy looking banco with concrete board and stuck thinset. I took the claw of my hammer and cut back the corners and edges about 1" of the cement board. I wanted rounded and soft corners.
I then mixed a 1/2 a bucket of plaster with some hot mud tossed in. Did the first coat and looked pretty good.
Second coat, I mixed almost 50 / 50 hot mud and plaster, within 20 min was splashing water on it and trowelling smooth ... Looks frigging gorgeous and the home owner loves it.
Two coats from a fugged up surface for ready to paint.
Thumbs up to you for using plaster, is just so many tricks to use it for, has such a nice finished look. It has such a strong body to it, you can bury the neighbours cat in the first coat. I love the stuff and wish more people used it.