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We started to prepare an old reinforced concrete ceiling of a panel building for painting. We cleaned off several layers of very old soviet wallpapers, glue, paint, putty and other stuff. Then we applied primer, then putty, primer again and then glued fiberglass(with Seinaliim glue). The next day fiberglass got yellow stains all over the place, everywhere. It seems like places where we applied more primer became more yellowish. And it's quite possible that I bought a wrong kind of primer. I was told to buy "Ceresit CT 17" but I bought "Ceresit CT 17 Pro" instead.

We were given advice that if we apply a new layer of another primer then we could fix it and then put some finisher putty and paint. And it won't be yellow.

The question is: Why it could happen and will extra layer of good primer help protect the paint from this yellowing?

Here is an image of the kind of fiberglass we used:

fiberglass photo

I don't think it's insulation, it's rather thin. The painter said we need it for the paint to look good.

I didn't make a photo of the result because the yellow spots are not vivid enough and won't be visible on my camera.

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  • What kind of fiberglass did you glue up? Was it sheets or raw strands (like insulation material) glued on as a mat or something else? I'm having trouble trying to envision this. Maybe include a good picture or two to help us out. Also, you mention "very old Soviet wallpapers" - purely out of curiosity, and if you're willing to share (since I don't think it will impact the answer), where in the world are you?
    – FreeMan
    Commented Mar 18, 2021 at 14:10
  • @FreeMan, It's Russia. I thought it would impact the answer because their quality was particularly low. I think the old putty contained chalk. And there was a bright red paint too under the wallpaper. We cleaned it all off but I worry that the remains of these materials might have done some damage
    – Gherman
    Commented Mar 18, 2021 at 16:57

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You will probably need a shellac based primer. It sounds like the coloring in the insulation is bleeding through.

I have found that shellac based primers are the only way to stop bleed through with other painting projects.

Zinser, bin and kill are the primer brands that work well for me. I have no association with these products other than being a satisfied customer.

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  • This fiberglass doesn't seem to have coloring and I think it's not insulation. Maybe I used the wrong term?
    – Gherman
    Commented Mar 18, 2021 at 16:54
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    Whatever is causing the bleeding this primer will stop that. The fiberglass mesh on a roll is to help hide imperfections in the concrete ceiling, that helps that you added that photo.
    – Ed Beal
    Commented Mar 18, 2021 at 17:02
  • I would suspect that the fiberglass cloth with also help to reinforce the very old concrete to ensure it doesn't flake off and fall on people.
    – FreeMan
    Commented Mar 18, 2021 at 17:43

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