I'm confused about the recommendation that "The primer must not form a closed, shiny film" found in quite a few primers sold in the EU as basis for emulsion (US ="latex") paints, e.g. in Caparol Tiefgrund. Why is film-forming a bad thing? I thought that was the purpose of the primer.
I've checked the German version and it's not a translation error: "Die Grundierung darf
keinen geschlossenen, glänzenden Film bilden". If you google this last phrase, you'll find lots of hits in other (some non-Caparol) primers, so it's unlikely to be a mistake.
Best Answer
A glossy finish is Bad News on a surface you are going to recoat.
Paint bonds to the underlying surface one of two ways:
Gloss and tooth are contradictions of each other. Gloss requires a mirror smooth surface, which a glossy topcoat is designed to give you.
In even the best paint systems, primer and paint are often chemically dissimilar. Therefore you rely on the mechanical bond. Primers are designed to leave a rough surface when applied normally, as a timesaver. If you managed to lay down primer in a glossy surface, you'd need to "scuff sand" it to create the tooth needed for the next layer.
But a glossy primer would also raise the question of whether you applied it correctly, and by extension whether it would cure and perform properly under those circumstances. For instance if you put it on too thick, you might want to give it a few more days to cure.