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I'm not sure what it is, but I've cleaned it with mould remover and it comes back after a few weeks.

Enter image description here

Peter Mortensen
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Annabel
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    How long have you lived in the property? It's hard to tell from the photograph, but that looks like chemical residue. – End Anti-Semitic Hate Oct 15 '23 at 09:58
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    Do you own or rent? – FreeMan Oct 15 '23 at 11:41
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    What's the wall made of? What is on the other side? – Criggie Oct 15 '23 at 20:54
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    Can you clarify if the status of the white substance we see in the picture is its "natural" status, i.e. it looks like this by itself, or if this is after you tried to clean it? – Vladimir Cravero Oct 16 '23 at 08:07
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    Is it damp? Have you tried something like "Do It Yourself Mold Test Kit" before using mold remover? And what kind of remover are you using? Chlorine based? – Mołot Oct 16 '23 at 08:49
  • This looks similar to the inside of a car windscreen when you wipe it with your hand or a dirty rag. The next time it fogs up, the residue on the surface attracts more of the condensation or "fog". What you might be seeing is the residue of the cleaning fluid perhaps mixing with the underlying cause. Perhaps it is the cleaning chemical itself, reacting with the paint or the plaster under the paint? What did you clean it with? – Chris Schaller Oct 16 '23 at 13:42
  • It's efflorescence ,it's caused by the moisture of the scheme coat trapped underneath the finishing coat. That's why it looks like man- made residue. – user186113 Mar 29 '24 at 03:17

1 Answers1

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Hard to be certain, but it looks like it might be efflorescence, which is caused by water passing through the wall and then evaporating on the surface. The minerals contained within the water are left behind, usually looking like a white powder on the surface.

The Wikipedia article suggests some countermeasures. My local home center sells an efflorescence remover that is basically diluted stearic acid. This dissolves the white stuff, but to address the problem permanently you need to eliminate the water.

bubba
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    Not disagreeing at all, but wondering, if it is efflorescence, why does the residue have such a non-organic shape? The shape of the residue looks human-made, not organic. – End Anti-Semitic Hate Oct 16 '23 at 02:13
  • @EndAnti-SemiticHate previous human efforts to clean it? – stannius Oct 16 '23 at 02:35
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    Are you sure it's efflorescence? When I search for images of efflorescence it gives results that look quite different (the image in the question looks almost like a powder [not unlike the residue you describe], but the image search shows little non-uniform bubbles). Search results here. – stevec Oct 16 '23 at 05:03
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    I do not think this is efflorescence at all. Usually you will find it at the bottom of walls, where the other side is underground or very close to ground, and most definitely it usually does not occur in the shape of sponge streaks. – Vladimir Cravero Oct 16 '23 at 08:06
  • I've retracted my up vote: The question asks how to remove it. The answer says what it is. There may be removal instructions at the link, but there are no instructions here. Without that, this is simply a link-only answer. Please summarize at least some of the instructions on how to remove it - we all know that links die and who knows what the next edit to the Wiki page will do to it... – FreeMan Oct 16 '23 at 16:26
  • @stevec — no, of course I’m not sure, and my answer doesn’t imply that I’m sure. I’m just making a suggestion based purely on the photograph and my own experiences with efflorescence. It goes without saying that other ideas are welcome. – bubba Oct 17 '23 at 06:15