ᐅ The plaster has come loose around the channels cut for wiring.

The substrate must always be properly prepared. For Poroton or Ytong, less bonding primer is needed, but a deep primer is required to seal the highly absorbent material. The bonding is ensured by the relatively rough surface.

Other substrates may not absorb as much, so a bonding primer is applied to improve adhesion. This is typically done when tiling over existing tiles. In this case, the bonding primer is essential.

Or for the exterior base coat (render) on facades. This also requires a bonding primer. Usually, quartz sand and deep primer are combined to ensure proper adhesion and to act as a sealant in the base coat.

However, I would say that treating the Poroton substrate with deep primer is absolutely necessary when applying gypsum plaster. Otherwise, just look at your bathroom—plaster tends to come off again.

Moreover, deep primer is not expensive. You don’t need a brand-name product. Ten liters (2.6 gallons) might cost around 12 euros.

Buchsbaum schrieb:

The substrate always needs to be properly prepared. With Poroton or Ytong blocks, less primer is needed, but a deep primer is required to seal the highly absorbent material. The adhesion is ensured by the relatively rough surface.

Other substrates may not absorb as much, so a bonding primer is applied to improve adhesion. This is common when tiling over existing tiles. In that case, the bonding primer is important.

Or for external plaster on facades. These also require a bonding primer. Usually, a mixture of quartz sand and deep primer is used to provide good adhesion and seal the base plaster.

However, I would say that treating the Poroton substrate with deep primer before applying gypsum plaster is absolutely necessary. Otherwise, just look at your bathroom — the plaster will eventually fall off again.

Moreover, deep primer isn’t expensive. You don’t need a premium brand. Ten liters might cost around 12 euros.

Thank you for the information. Unfortunately, I had no influence on these details since I am building with a turnkey contractor. When a so-called master company with extensive experience carries out the work, as a buyer you assume the people know what they are doing. Somehow the supposed savings are not justified when everything comes off...

It's not a big deal; the plaster will hold as long as no one is hammering around there.

You can test yourself whether a primer/sealing coat is applied on the Poroton bricks. As mentioned, the easiest way is to spray water on the bricks using a spray bottle filled with water (like a glass cleaner bottle, etc.). If the water is immediately absorbed in a spongy manner, then there's no coating. If after several sprays the water beads off, then a sealing layer is present.

Hunsrückblick schrieb:

Thanks for the info. Is the use of a primer for plaster specified in any standard?

Why exactly the plaster reliably fails during the warranty period, like clockwork, is "secondary" ;-)

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Buchsbaum schrieb:

It’s not a big issue; the plaster will hold as long as no one is hammering on it.

Try testing yourself whether there is a primer on the Poroton. As mentioned, the easiest way is to use a spray bottle filled with water (window cleaner bottle, etc.) and spray it onto the blocks. If the water is immediately absorbed in a sponge-like manner, there is no primer. If it beads off after several sprayings, then a barrier is present.

I just went around the house and used the spray bottle. The water soaks into the blocks somewhat, so there probably isn’t any plaster primer on them. I hope this doesn’t develop into a bigger problem and is limited to this area. Soon, I will carefully tap the surfaces with an expert for further investigation. Thanks for the information.