Painting works for prefabricated house: Which interior paint? Painter's fleece?

If it is pure silicate paint and not dispersion silicate paint, it is already more difficult to process. In this respect, a surcharge beyond the pure material cost increase is justified. Whether now 3.80 /m2, I cannot say.

can this be explained by anything other than rip-off?

Is that the business model? The big money is not made with the basics, but with the upgrades. Applies to houses, cars, vacations,...

Ideally, you think about it beforehand and have something like this written into the offer phase or have the trade removed. Then, in case of doubt, you still have the chance to look for another general contractor. In practice, it often doesn’t work out ideally. Whether the additional charge is justified or not is also completely irrelevant. Either you stick with the standard, pay the surcharge, or have the painting work removed from the contract now and assign it yourself. In option 3, however, the planned profit definitely remains with the general contractor ;)

Does silicate paint have any disadvantages besides the price, and what surcharge would be justified? I find a surcharge of €3.8/sqm from standard dispersion paint to certified dispersion paint really steep, or can that somehow be explained other than as rip-off? I looked up both paints in common online shops and the difference is only about €1.5/liter, which would maybe amount to 50 cents per sqm...

Then get the paint and paint it yourself ;)

The painter or the general contractor can charge whatever they want, you don't have to buy it.

If you're already having Q3 plastering done, take dispersion silicate and skip the painter's fleece. That saves money.