Now I'm venturing into deep water.
Our painter

plastered a wall where we tore down about 2dm for a larger opening.
Boards on both sides of what needs plastering, and depending on the depth, fill with brick and use mortar intended for plaster/masonry "depth".
If the long sides of the boards are straight, you get an even and straight side, "just smooth along with it."
There are certainly better ways, but this turned out very nicely for us... But then the house is not entirely angle-secured
Apparently, it didn't exist in 1923...
