Rendering base or brush "clean"?

[kent_n]

Living in a house from 1956, a one-story house with a basement.

Last week, I sanded down all the basement walls in our basement. I used a Flex concrete grinder. This was to remove the paint that was applied on all the walls.

Now there is a rather porous plaster surface on the walls, which can be brushed away with a steel brush or similar. Under the plaster, as far as I understand, there are concrete blocks. The plaster isn't loose anywhere but is, as mentioned, quite porous.

Now to the eternal question that I haven't managed to find a sensible answer to. Does the plaster need to be sanded away, or is it enough to use some kind of primer (like red primer) before we start plastering the walls again?

Have you done the same type of renovation? How did you do it? Have you realized afterward that you did it wrong?

Incredibly grateful to hear about your experiences.

 

[Helioz]

If it sands so much that you can easily scrape or wire brush everything off, it must be removed in my opinion. But if it's still solid and nice, you can apply a slightly thinner layer (5mm) with Putsbruk C 0-1mm, which you float instead of using a scrub board; this works well when there is a porous surface underneath. This plaster isn’t thrown on but is preferably mixed a bit looser than usual and applied with a steel trowel. Then let it dry for 15-45 minutes depending on the temperature, and finally float the surface with a damp float.