8 Nov 2016
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United Kingdom

I hope someone can help. We recently had a hallway replastered, following a damp course. I applied a mist coat of watered down emulsion, and it has come up with a mapping effect - that is to say big raised patches where the mist coat hasn’t properly sunk into the walls. I somewhat stupidly started painting over this with the topcoat, before I realised what it was.

Do I sand the wall back, and then re-apply Mist coat? If so do I have to take it back to the plaster? Should I seal the wall? If so, what with and do I have to use the sealer on bare plaster or just on top of what’s there?

There’s a o much conflicting advice out there!
Thank you
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Any chance of a picture or two? It's hard to tell exactly what the problem is, or advise on a fix, without seeing it.
What do you mean by 'raised'...rough, bubbled, blistered??
Any chance of a picture or two? It's hard to tell exactly what the problem is, or advise on a fix, without seeing it.
What do you mean by 'raised'...rough, bubbled, blistered??

Whoops! Sorry yes!
Here are pics (brown is where I top coated)


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Wow! :eek: That's the most pronounced case of mapping I've seen, on freshly plastered walls. If that had happened when I'd painted it, I'd have called the damp course installer and the plasterer back, if only for their opinions as to what the cause of the issue is.

As a decorator of 20+ years, I can confirm that we are infamous for complaining about plasterers, but I can't see how that should have happened with fresh plaster unless the dampcourse hasn't been done correctly and there is still damp in the wall or the plaster and render wasn't fully dry before you painted. Even then, those raised areas almost look as if paint has been scraped off in patches and painted over, rather than it being new plaster with a moisture issue.
Was it hacked off back to the brick and, if so, how long was it left to dry, or was it just skimmed over the existing surface? I'm hoping it wasn't the latter, because damp walls really need to be hacked off in order to be treated and remedied correctly for decoration.

If it's as bad as the images make it look (Maybe you've just got a super high resolution camera which is making it appear worse?) I wouldn't be happy with anything less than a re-skim. Sanding, filling and repainting is a lot of work to be doing on a brand new surface, which shouldn't ever be necessary.
If you have painted it too soon after plastering, I'm afraid its just a case of biting the bullet and getting all that work done yourself.
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Thanks for this. It was taken back to brick and left for 3 months to dry. The damp proofer has come back and said it’s efflorescent salts (?!) and will be covered by another layer of paint. Considering that there are two layers already on there, I’m not at all convinced but I’ll try it.

I’m assuming they’re not going to come back, so I think it’ll be left to me to rectify. There are several thick hairline cracks which keep reappearing too.

Any ideas? I’m thinking of a thick basecoat first to help cover my terrible filing and sanding. Any suggestions?
I was going to mention efflorescence but only the second image gave me a slight suggestion that this might be the issue - the first image certainly doesn't look like it.
As the damp proofer has come back and told you that's what it is, I'd have to believe him because the camera does sometimes lie!

Another coat of regular paint won't necessarily cover the problem and you should consider a coat of good quality alkali-resistant primer. This product is specifically designed for this type of problem surface, but is not always 100% successful because some residual moisture can stay in the walls for years. Zinsser Bullseye 1-2-3 PLUS also has alkali resisting properties, so you could use that instead, but it is a combination primer and not specifically designed for the job in hand.

Before using either, you must ensure that all moisture within the wall has gone and that the surface is dry. Sand, scrape and brush away any rough areas and salts, then apply the ARP. Once this is dry, you can do any fine filling and sanding that is necessary. Filling before applying ARP introduces moisture to the surface which can cause the salts to bloom again, and you want to avoid any form of moisture coming into contact with the dry surface before the primer. After the filler has dried, carefully sand it smooth, to avoid damaging the primed surface. You are then able to paint with your chosen emulsion, lightly mist coating the filler, if required.

It sounds like a lot of work, but it's not really too bad once you get going.

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