Primer applied on waterproof putty became flaky and peeled off

[Wrichik Basu]

A section of the wall in our house developed flakiness due to dampness, prompting us to undertake repairs. Specifically, the affected area was located behind an air conditioner. It appeared that poor initial plastering, combined with water leakage from the old AC indoor unit, caused the concrete to become flaky. Consequently, the old plaster was removed, exposing the brickwork, and a new layer of plaster was applied by a professional mason. Once this was completed, I proceeded to apply a layer of waterproof putty, specifically, this product.

After allowing the putty to dry, I noticed certain areas that appeared thin, so I decided to apply an additional layer without sanding the surface. My intention was to maintain the layer's thickness. Although the resulting surface was not perfectly smooth, it wasn't of great concern since it was a wall hidden behind an air conditioner. The main objective was to protect it from dampness for as long as possible.

Once the second layer of putty dried, I proceeded to apply a waterproof primer. I used this particular primer, which, according to the technical data sheet, could be thinned with water. However, it did not explicitly state whether dilution was necessary. To clarify, I consulted with the shop owner from whom I purchased the primer, and he confirmed that thinning the primer was not strictly required.

Consequently, I applied the primer as is, without diluting it. As it began to dry, the wall started to exhibit flakiness:

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Before long, the entire wall peeled off. I believe the primer removed a layer of putty as well. All labour lost.

In order to rectify the situation, I will need to apply another layer of primer. However, before proceeding, I need to understand why the primer peeled off. Do you have any insights into the possible reasons behind this issue? I believe I have to thin the primer, right?

 

[jrmichler]

Just guessing here. According to the paint data sheet, the substrate must:

1) Have less than 12% moisture
and
2) pH between 7 and 11

The data sheet also mentions water resistance. One possibility is that the putty still has high moisture content, and possibly high or low pH. It is also possible that moisture is coming through from behind the putty. If so, the moisture in the putty can push the paint off. I have seen a mural painted on a brick building peel off exactly like in your photos. That building was part of a paper mill, and had high moisture inside.

Joseph Lstiburek has done a lot of research into moisture flow in walls. His web site has a lot of good information in the subject. This link is to just one article on that web site: https://buildingscience.com/documents/building-science-insights/bsi-135-throttling-flow.

A remedy would be to remove all of the primer, and repaint with a moisture permeable paint. Most water base latex paints are moisture permeable. Put on just enough paint to get coverage because too much paint makes it less permeable, and more likely to flake off.

 

[Wrichik Basu]

@jrmichler Thank you for the reply. It is possible that the upper layer of putty had not dried and cured fully, which is why when the primer dried and shrunk, the top layer came off.

I doubt that moisture can come from behind the putty in this wall. There are no water pipes; it's nearly 40°C for the past few days, with humidity quite low as well. The external wall is painted properly. No rain in the past few weeks either.

For the while, I have not applied another coat of primer, primarily out of fear that it may scrape off the lower layer of putty. Some portions of the wall still have the primer; some don't. We will leave it like that for the moment. We will not be painting the surface because of budget reasons.