Gypsum board on a sloping roof, apply felt plaster.

Then I will ask the "Bautechniker und Gipser" I commissioned on-site for professional advice.

So, after I got in touch with the company Schäfer, a friendly gentleman told me on the phone that everything regarding the components Filzputz 600 and Sperrgrund is obsolete. Gypsum and cement should not be combined; instead, a quartz primer should be used for painting and then 5mm Pro Kalk Reno, fiber-reinforced, should be applied.

Strange argumentation. The representative surely wouldn't want to claim that his Pro Kalk Reno contains no cement at all...
Moreover, the GK gypsum is virtually material-decoupled by the green colored paper; the gypsum does not touch the plaster anywhere.

For the GK primer, the technical data sheet also prescribes Sperrgrund LF and not (only) quartz primer – the gentleman should also study his section "Putzgrund" and not stop reading after the heading.

But as already mentioned, in my opinion the described risks remain. I wish you that your thin plaster will still look good in a year, despite the enormous vapor load.

They are not entirely in agreement in the datasheet either; once a quartz primer is sufficient, and further down a sealing primer should be used. I will use the sealing primer, as according to the processing guidelines for [ProKalkReno] that should be enough. I therefore find two quartz coatings unnecessary.

I think the reason for blocking for GK is meant. The technical data sheet also directly addresses the discoloration issue. If you have the "Gipser" do the plastering, also leave the surface preparation to him. Because if his work peels off next year, it will be your fault since the substrate was wrong.

Hello, so the "Gipser" was on site, but he can only "treat surfaces," he can't apply plaster, lime plaster is a foreign word to him, and I won't be talked into anything, I'd rather wait a bit until a plasterer who can do that has time and, above all, has proper advice ready.