Loose corner joint

[moaxed]

Hi!
I have built a custom workbench in a kitchen and now need to solve a corner joint where a demolished wall used to be.
I want a nice wall up to the plywood and want to avoid moldings/sockets.

What filler for the holes would you recommend?
I'm nervous about using foam that expands and puts pressure on the wood.
What alternatives are there? The final finish is intended to be done with fiberglass fabric/putty, but need something suitable to attach the putty to.
Best regards,
David

 

[jonko]

Plaster is your friend, it can be mixed like putty and filled generously in one go, and any excess is easy to sand down.

 

[moaxed]

Gypsum plaster is your friend, it can be mixed like putty and filled generously in one sweep, and any excess is easy to sand down.

Sounds very suitable!
Is it "injectable"? Some holes are so deep (but narrow) that it's difficult to get in with a trowel.

 

[jonko]

Sounds very suitable!
Is it "injectable"? Some holes are so deep (but narrow) that it's hard to get in with a spade.

No, but the question is whether it's necessary? Otherwise, maybe you can use a narrow pointing trowel to get mortar in where it's really narrow.