Basement walls

[C_jay]

Hello
I'm working on getting a room in the basement in order.
The room was divided by a wall which I have torn down, and then I removed the fabric that was on the walls. The plan was to take everything down and then repaint everything in some open color.

Now we come to the problem. Old paint was left on the plaster. I have tried to pick off, sand, and scrape with a steel brush. The only thing that happens is that I dig into the plaster around, but the paint remains.
The plaster is very porous and gets holes just by poking, basically.

Do I have to knock off the plaster and re-plaster?

 

[C_jay]

Update

You are who you are. Impatient as few.
So I started scraping with a paint scraper and it goes fairly well but a lot of plaster comes off..

If you continue like this the question is if there is a special plaster you can apply on the old one? Or should you remove all the old plaster?

 

[Stefan N]

There is 1mm puts that can be used.

 

[C_jay]

Is it at least 1mm thick when laying it down? Because I think it is deeper in some places..