How should you fix the seam in the wall?

[Smoothie]

I am going to close up a doorway. The walls are from 1939 and contain boards, reeds, and plaster. On top is a sort of wallpaper-like material.

For the doorway itself, I thought about framing and using OSB and drywall to make it even.

But I'm unsure about the seam that will form between the new stable part and the old wall. The outer layer closest to the seam in the old wall gives way when you press on it, and most of the plaster has fallen out.

How should one fill/fix the seam and make it sufficiently stable to get everything even and nice in the end?

Attaching pictures...

 

[Stefan N]

The only chance for it to stick is to plaster the entire wall.

 

[Smoothie]

The only chance for it to last is to drywall the entire wall.

Do you mean all the way up? (How do you deal with the seams on the sides?)

Or do you mean applying something like repair drywall over the existing wall, plus some thicker drywall in the doorway?

 

[Stefan N]

The latter, fill the hole and plaster the entire wall.

 

[Smoothie]

Is there perhaps an alternative solution? Other than plastering two whole walls?

For example, "Reparation och kakelfix PF420"? And some adhesive if needed?

https://www.jula.se/catalog/bygg-oc...h-spackel/fog/reparation-och-kakelfix-502049/

(It will likely end with renovation plaster at a later stage, at least on one wall where old kitchen cabinets will be taken down.)

 

[Hans G2]

When I tore down part of a wall, I repaired similar damages with plaster, and although it's only been a year, there have been no cracks so far.

 

[Smoothie]

It turned out quite well in the end, after a lot of fixing and filling. On the other side (the kitchen), renovation gypsum will be used instead.