Plaster ceiling

What's the protocol on plaster a ceiling? I won't get into specifics of how this happened, but basically the ceiling has drywall that has a 4-5 inch gap between sheets that runs along the entire length of a couple of sheets (say 10 ft long). Previous owner did a bad job covering the hole with plaster and now that part is extremely uneven. Should the old plaster be removed and started all over or can the new plaster be applied on top of the old plaster? Also, is sanding to even out the plaster in between the two drywall sheets a must? Look forward to comments as not crazy about how my painter/plaster guy is doing so far (he’s applying plaster directly on top of old and creating a bit of a bow between the two sheets of drywall, which I think will eventually lead to a crack).