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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.

  • I'm guessing then that the German wiki page on the generic product (de.wikipedia.org/wiki/Tiefgrund) is somewhat wrong, as it says to apply to saturation. To counterbalance that, it also says that shiny puddles are bad for adhesion of subsequent products. – Fizz Jun 25 '17 at 18:18
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A glossy finish is Bad News on a surface you are going to recoat.

Paint bonds to the underlying surface one of two ways:

  • the next coat of paint is chemically compatible with the last coat (usually that means the exact same paint), and the last coat is still chemically active enough for a chemical bond to occur. If you read paint cans, that's what "recoat time" is all about.
  • the surface is microscopically rough, i.e. has lots of jaggy mountains and canyons. This is called tooth. A surface which has been recently sanded will have this characteristic. The next coat will form a mechanical bond by the drying paint interlocking with the dry paint.

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.

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That means don't put it on too thick.

Primer is for providing a bonding layer for the topcoat and to cease absorption into the substrate.

Primer lacks the amount of binder topcoat has. If you apply primer so thick that it can skim over and form a film, underneath that will cure to sub-par 'coat' of paint.

The paint type known as Emulsion in the UK and Latex in the United States is a water-borne dispersion of sub-micrometer polymer particles. These terms in their respective countries cover all paints that use synthetic polymers such as acrylic, vinyl acrylic (PVA), styrene acrylic, etc. as binders. The term "latex" in the context of paint in the United States simply means an aqueous dispersion; latex rubber from the rubber tree is not an ingredient. These dispersions are prepared by emulsion polymerization.

Such paints cure by a process called coalescence where first the water, and then the trace, or coalescing, solvent, evaporate and draw together and soften the binder particles and fuse them together into irreversibly bound networked structures, so that the paint cannot redissolve in the solvent/water that originally carried it. The residual surfactants in paint, as well as hydrolytic effects with some polymers cause the paint to remain susceptible to softening and, over time, degradation by water.

The general term of latex paint is usually used in the United States, while the term emulsion paint is used for the same products in the UK and the term latex paint is not used at all.

  • It;s actually hard to put them "too thick" as these have at most 1g/cm3 density, like water basically, e.g. (OptiGrund)[caparol.de/uploads/pics/caparol_import/caparol_de/ti/21207/…. The one I've linked to in my question has even less at 0.8 g/cm3 as it uses mineral spirit as solvent. What one must is dilute them, with the appropriate solvent; for most it's water. – Fizz Jun 25 '17 at 17:46
  • It could be in reference to this: Should I use oil primer underneath oil paint? : "you can use latex paints over oil primers and latex primers. Do not use oil based paints over any surface currently coated with a latex primer or paint. The oil based paint or primer will usually lift the latex product and leave you with a wrinkled surface that looks like alligator skin." – Mazura Jun 25 '17 at 18:03
  • But I don't think so. It's hard to, but if you dip it all the way in, and you don't wipe your brush at all, you can put it on too thick. Especially if you're expecting it to go on like a real coat of paint. – Mazura Jun 25 '17 at 18:04
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Primer should be applied lightly, your not painting the room just adding a adhesion coat. Quick tip you can always tell the better primers and paints by weight. Try picking up a gallon of cheap paint and good paint in each hand, it's because of the solids content of the paint good paints have lots of solids which are whats left after the paint cures. While cheap paints have less material that actually stays and doesn't evaporate. Most cheapo paints by the same manufacturer usually are the same ones just watered down.

  • Actually, in my experience, the best paints have solids somewhere in the middle around 1.5 g/cm3. Those with lots of solids (say 1.67 g/cm3) tend to have more calcium carbonate which is a cheap filler and has little covering power compared to titanium dioxide, which you don't need as much and is more expensive. – Fizz Jun 26 '17 at 14:02

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