How clean should a concrete block wall be before plastering?

[aekeberg]

Planning to plaster a room in the basement. How thoroughly must the old paint be removed? See videos below for how it looks... Which plaster should be used? Need to apply approximately 10mm.

Flakes off in places and I feel like there is a powder that has crystallized/collected behind (apologies for the poor camera work):

Then it sticks very well in patches:

Related to our basement project that I describe here: https://www.byggahus.se/forum/threads/ytterligare-ett-kaellarprojekt-villa-fran-34.363717/page-6

//Anders

 

[aekeberg]

No one who has some input to give?

 

[Claes Sörmland]

If it is regular latex paint (acrylic paint) for indoor use, the wall is quite impermeable to water vapor. This can be a problem if moisture needs to dry inward from the wall. It can also explain the peeling paint in some areas. From that perspective, the paint should be removed before plastering with C-mortar and repainting with silicate paint (or lime paint).

A perhaps slightly less orthodox view could be that despite being impermeable, the paint has worked well. It's just that nothing lasts forever, which is why it is peeling in some places. From that perspective, you might consider plastering over it. It should work with gypsum mortar. But a special primer with sand in it is needed for the paint to adhere to the paint. Here is an example of a plaster system:

https://www.se.weber/gor-det-sjalv/putsa/putsa-inomhus

 

[aekeberg]

If it is regular latex paint (acrylic paint) for indoor use, the wall is quite impermeable to water vapor. This can be a problem if moisture needs to dry from the wall inward. It can also explain the paint peeling in some places. From that perspective, the paint should be removed before plastering with C-bruk and repainting with silicate paint (or lime paint).

A perhaps somewhat less orthodox view could be that the paint, despite being impermeable, has worked well. It's just that nothing lasts forever, that's why it's peeling in some places. From that perspective, you might consider plastering over it. It should work with gypsum plaster. But a special primer with sand in it is needed for the plaster to adhere to the paint. Here is an example of a plaster system:

[link]

Visited the neighbor across the street today. He had painted his basement with silicate paint. I must say it reminded me a lot of my wall, albeit ten to fifteen years younger!

Can silicate paint also peel 'in flakes'? I imagined it would become more a part of the plaster but maybe I got that wrong...