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:
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.