Comprehension question: paint directly on plaster

We did not sand our Q2 gypsum plaster at all, but instead primed and applied paint. However, it also only had few even textures, and that was how we wanted it.

Yes, you probably sanded a bit too much there. No wonder with 80 grit sanding paper. We also finished our gypsum plaster ourselves with a giraffe sander and I think 160 grit, primed it and painted it with biosilicate paint. It looks great. Dents etc. were filled with gypsum filler beforehand.

First of all, thanks a lot for your advice. Yes, 80 grit is too coarse, but everything can be fixed. I smoothed it today with fine filler. You can't see it anymore. Since I couldn't sleep peacefully anymore, I also asked a friend for advice. He comes from the construction field. I'll write it down here, maybe it's helpful for the next person, what I was told: you need a smoothing trowel, fine/smoothing filler (not more than 10€), then a hand board and sanding mesh with 180 or 240 grit. You can also add a dab of paint to your primer ([Tiefengrund]). That way you can see better while priming where there are voids or dents. Maybe you don't even have to scrape the whole wall. Better to sand by hand - without a giraffe [Schleifgerät]. The black little stones are just the mineral content in the plaster. If I had known right away that there is something like Q2 to Q4 for plaster. Nothing like that was written in my construction contract, it just said "smoothly scraped off." Wallpapering is not so good, it doesn’t let the wall breathe and it takes quite a while until everything is dry. For me, it’s 6m³ of self-leveling screed... Best regards & have a nice Sunday to you.

One can also add a dab of paint to their primer (deep primer).

Not "one can" - every DiY enthusiast should do this - a cup of paint into a bucket of primer; this makes the final coat adhere better. If the chosen paint is a branded product from a specialist retailer - not a hardware store + co. - this clever approach often saves the second coat. Rhineland greetings

. Wallpapering is not that great, the wall can't breathe and it takes a while until everything is dry. For me, it's 6m³ of flowing screed... Best regards & have a nice Sunday

Walls are dead, they don't breathe anymore anyway (I always can't help but say that). On the other hand, wallpapers are very permeable to moisture. from on the road

Yes, could be. The master painter was at my place today. He said that in Q2 everything would have to be filled/smoothed. Then he would recommend some kind of renovation fleece base and a silicate resin paint. Of course, sanding twice, priming, etc. etc.
I will still get the offer price. But I think I have to do it myself. I had too many additional claims during the construction ;-(
Besides, there’s another damage:
I’m just saying, condensation/sweat water on the floor slab...