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I've started to paint my basement with white primer, and on one wall weird orange spots appeared in the holes I had "filled" with the primer.

The wall is made of dark cinder blocks which are pretty rough. I have to use a lot of primer in general with the roller and need to touch up with a paintbrush all the major holes to fill them.

At some point during painting I noticed the yellowing in one hole, but didn't think much of it and filled it again.

After 3-4 hours I checked on the wall again and saw 2 dozens such spots in a bright orange color, much like rust. One other wall isn't affected at all, but the primer I used was a different brand. Now I'm using a new KILZ 2. Before, I was finishing a Benjamin Moore white satin paint.

As you can see from the picture the hole is still "closed" by the paint.

Weird orange spot on freshly painted cinder blocks

In that section the wall is pretty dry. We've had some efflorescence near the ground (which I've removed), but not at all where I'm painting now. The wall is 25% above ground and the spots have appeared both above and underground.

The wall was also painted with just one layer 8 years ago on one section of the same wall before we bought the house, and I've now noticed the same spots.

Any idea what that could be? I'll be continuing with the painting over the next days.

Here's a picture of the unpainted cinder blocks. enter image description here

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    I'm thinking your concrete blocks have some bits of iron in them (not at all uncommon) that is being oxidized by the water and other chemicals in your primer. Try a variety of sealer/primer that specifically lists "rust inhibitor" as an ingredient.
    – jwh20
    Jan 6, 2020 at 16:27

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The orange stain is likely an iron ochre infiltration due to high levels of iron in the soil or pre-existing iron in the cinder block. More info

The paint is simply making the orange much more visible than it was on bare cinder block.

I highly advise reading this article on basement finishing before continuing to paint your basement. You will likely have to re-paint next year because moisture is merciless and you might even cause mold issues for yourself which weren't present before.

If you really want to make the cinder block look nicer then you can try staining it. More info

Below is a picture of my basement wall which was Drylok'd about 4 years ago by the previous owner. Note: it looked like new when we moved in and it quickly went to crap.

mold and rust on dry-locked basement wall

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  • Thank you. I under from this that the ochre color is indeed an infiltration, and that I shouldn't be painting the wall... that's going to disappoint the missus (and it does indeed look better painted...) Is there a way to paint while retaining some porosity that would allow the blocks to "breathe" ?
    – GCord
    Jan 6, 2020 at 18:49
  • @GCord Paint it or don't paint it; it's your house I won't lose any sleep over it. I just wanted you to understand the pros and cons of your choices.
    – MonkeyZeus
    Jan 6, 2020 at 18:53
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    @GCord Your Googling skills are probably on par with mine but I would imagine that raw cinder block looks better than the rust and eventual mold. If you're going to finish the basement then do so properly; until then every "solution" is just a stop-gap.
    – MonkeyZeus
    Jan 6, 2020 at 19:00
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    I don't think there is any sort of "porous painting method". A single block has thousands of pores and each one has the potential to allow water so if a blotch of paint is covering that pore then that blotch will blister and peel.
    – MonkeyZeus
    Jan 6, 2020 at 19:04
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    @GCord Now that I think about it you can try staining the cinder block. I updated my answer with a resource.
    – MonkeyZeus
    Jan 6, 2020 at 19:38
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The stains are minor, but may get worse over time due to moisture seeping through the porous concrete block from the outside.

In order to have a dry basement, it is usually necessary to waterproof the foundation from the outside. This is generally not a DIY project since it involves digging all the way down to the footing in order to access the foundation walls. If the foundation is not waterproofed from the outside, water will continue to seep in. When the foundation walls are waterproofed from the outside, it is common practice to put in a gravel base and pipes around the footings that will capture the water and divert it away from the foundation. This is known as a French drain system.

If the inside walls are continuously wet, painting over them with sealer is not a good idea. The water will saturate the concrete and not be able to dry out. Over time, this can cause the foundation walls to break down and cause more serious issues.

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