Spackling Q2 / 2 coats of paint - sufficient?

As far as I understand, it depends on the plaster.

Lime-cement plaster is diffusion-open and forms a bond with silicate paint – “silicification.” Silicate paint is inherently fungicidal due to its high pH level.

Dispersion paint is the “normal” paint, as I know it from the hardware store. It does not bond with the plaster but simply coats it. Fungicides are then added if necessary.

And there are mixed forms.

All clear. Thanks for the nice answer!

The silicate paint usually used indoors is the dispersible silicate paint. It can be applied throughout the house, even on, for example, gypsum board. According to the manufacturer, a primer should be applied depending on the substrate, but it also works without it. You just have to consider whether you prefer to have plastic on the wall or a predominantly mineral product. Both have advantages and disadvantages. Personally, I only have silicate paints in my house.

The trend towards baby-bottom smooth walls is completely incomprehensible to me...

Is that a trend?! I find it nicer when the walls are nicely even and smooth. With us, it was filled and sanded twice and is currently being painted. I personally find that really, really nice! A visible difference compared to our Q2 plaster.

I can expand: If you can actually live with irregularities and even want them (e.g. [Industry Style]), then Q2 is sufficient. If you expect that it will replace Q4 when done well, then take Q4 right away.

And additionally, just to prevent any misunderstanding: painting (even twice) practically does nothing to the structure of the wall. Only if the intention is to somewhat smooth out irregularities here and there. Q2 remains Q2. This means visible, even irregular, plaster marks, as this thread clearly shows again:

We wanted the ceiling to be completely smooth and the walls with a fine texture.

We had everything spackled to Q2 - then decided to have the ceiling fully spackled (2x) and then sprayed with paint. It just looks super cool and I would definitely recommend it anytime.

For the wall, I can also recommend painting everything with a base coat (Quartz base) and then applying a second layer of roll plaster (Deco Quartz). Our plasterer recommended painting everything again, which we did not do. It already looks amazing to us.