Walls were smooth at first, but after painting, there are grooves and marks

Hello everyone I want to have the walls in my house mostly painted and no wallpaper. I have gypsum plaster and have already done the first rooms. So far manually, now I have bought a long-neck sander. My question: after filling and sanding the walls are always really smooth. Then I prime and paint and afterwards I have really pronounced scratches and marks in the plaster. What am I doing wrong? Regards Enrico

Then they were probably not really smooth after filling and sanding.

It was obvious, there's always some stupid talker around.

But if the wall shows grooves after painting that weren't there before, it's not about whether the wall was perfectly smooth before or just a little bit, but about the fact that it has become more uneven, no matter how smooth it was before.

That sounds like it was wrongly filled (did not set properly). And then with the wet paint and the primer, the stuff came off the wall again.

I’m not talking nonsense, I’m just saying she wasn’t smooth. Scraping can’t make it coarse, polished putty has set. The only conclusion is, she wasn’t smooth.

Which primer was used and how was it applied?