Our kitchen has one of the main walls of our house which is a thick concrete wall. There is nothing behind the wall, just some empty space and then the neighbor's house. I just painted the wall with Flexa indoor water-based color, as grey.

Now, as you can see in the pictures, after a couple of months we see some green spots on the wall in different places.

enter image description here

enter image description here

My question is, what are these green spots? They don't look like mold or mildew.


  • I just tested our cleaning sprays on the wall to see if it happens because of some spray drops but it was not the case.
  • This place is not reachable for our kids.
  • 1
    Looks like some kind of deliberate marker
    – user253751
    Mar 5 at 14:17
  • Is this in an area where children might have splashed something on it like lime Kool-aid or colored with a marker? Mar 5 at 14:18
  • They are not in a reachable place for our kids and we have many of these spots.
    – S.Yavari
    Mar 5 at 14:18
  • 3
    That is a bold comment. I have a neighbor kid over here last week manage to get orange soda pop spray on everything within 4 feet. :) Mar 5 at 14:20
  • What is the actual wall surface? Obviously that's not bare concrete. Also, what's the scale of those photos?
    – isherwood
    Mar 5 at 14:25

Looks like copper corrosion products - so if there is a bit of copper embedded inthe wall, this sort of staining to the surface is quite plausible. WHY there's copper there to cause staining, I can't tell you.

  • Maybe someone was pressure-treating wood in the area and for one reason or another it splashed?
    – MonkeyZeus
    Mar 5 at 19:23
  • Pressure treating is not a DIY operation and most of the DIY preservatives don't have copper, IME. I'm thinking "copper nails" but having a hard time figuring why anyone would have put copper nails in a masonry wall, so I'm back to I Don't Know but Looks Like Copper...
    – Ecnerwal
    Mar 6 at 12:56
  • I certainly did not mean to imply industrial pressure treatment. I was implying the DIY route with a Copper Naphthenate application. I wasn't sure if "preserving wood" is the proper explanation of using such a product so I figured "pressure-treating" would be more universally understood.
    – MonkeyZeus
    Mar 8 at 16:38

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.