My basement is having some water issues. I am working to resolve that, but in the meanwhile: can you guess what is causing this pattern in the painted cinder blocks near the bottom right corner (some at bottom left as well)?

What I am getting at: are the edges of the blocks a darker color because they were wet when they were painted, or because water came along after they were painted and did something?

enter image description here

3 Answers 3


The darker color indicates the infiltration of water. It usually occurs after the block was painted rather than prior because, as usually the case, the paint wouldn't adhere to a moist/wet surface.

However, it can occur in a basement if the wall-floor edges were not cleaned properly (the surface was dry but dirty).

  • I had a basement drainage person out yesterday, so I grilled him about these patterns as well. He suggested that the blocks were dry locked in the past, and then some water forced itself through the grout causing discoloration. Then the former owner ground out the discolored areas (e.g. mainly the grout and nearby) and then painted everything. So the edges that were cleaned out are a bit darker because more of the paint was absorbed by the cinder block.
    – negacao
    Jul 13, 2021 at 15:18

Cinderblocks/concrete blocks normally have a hollow section:

single core block example

If they were mounted sideways, so that the core was in the middle, then they would need to be filled in with concrete and they would end up being different, even when finished & painted, in the middle than on the sides.

This does seem a little odd, because normally they will be stacked so that the hollow core is vertical - i.e., so the wall sides are solid. But it could explain the problem.

  • 2
    It's a reasonable explanation except that the phenomenon is not consistent and that block, as you mention, is not installed flat like that. I'd be interested to know if there is ever a reason (beyond wonky DIY job) to install them on the flat.
    – FreeMan
    Jul 12, 2021 at 16:07

My guess was going to be that the mortar is more porous than the cinder blocks so there was more water infiltration coming in those areas and affecting the color regardless of whether the discoloration was done during or after painting.

However, upon closer inspection I can see that certain blocks are fully affected by the discoloration so that leads me to believe some sort of sealant was used prior to painting.

Most discoloration seems confined to the mortar and surrounding areas which is akin to the application results of grout sealant.

In particular, the left block seems fully affected but the right block was only half affected so the previous person just applied it more liberally than other spots for whatever reason.

enter image description here


It's relieving to see that my basement walls aren't the only ones that look less-than-appealing =)


  • Hah thanks for the link, that was very informative. I am also cursing the former owner for making this mess, rather than dealing with the problem properly. Once I am satisfied with the exterior drainage and sealing, I think I'm going to get all this paint and crap off. Have you ended up trying anything with your basement?
    – negacao
    Jul 12, 2021 at 15:28
  • @negacao I have not tried anything with my basement. Having a good dehumidifier with an automatic pump (~$300) prevents mustiness and has prevented new black spots from forming so I make sure to keep it operational. I have a 2 and 3 year old so clearing out the basement to try and scrape the Drylok is wayyyyyy down low on my priority list. Realistically, I'll just clean it up and Drylok it again when we're ready to sell the house ¯\_(ツ)_/¯
    – MonkeyZeus
    Jul 12, 2021 at 15:34

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