Dark spots under newly painted plaster - HELP!

28 Oct 2010
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United Kingdom
I recently got round to painting a newly skimmed bedroom. After giving first coat of diluted dulux supermatt... patches of the paster do not take the paint and appear darker. Tried a second coat on these areas and it wont go away.
Can anyone suggest what has happened.. gone wrong!

The ceiling were also done but appear perfect.. so just the walls...
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Sounds at this stage that the skim wasnt fully dry when you applied your paint, tell me was the skim pink and uniform in colour or did it have those dark patches before you started to paint.

Hot air rises so it would be possible that your ceiling was dry and yet your walls were not.

The plaster was left to dry for about 5 weeks.. so pretty sure it was dry. And no the dark spots only became visable after the paint was applied. It was almost like the paint did not want to stick to certain parts of the plaster.
The second coat is the same.
are the dark patches brick-shaped, or is the wall near a possible source of water, including an adjacent bathroom or downpipe, or a windowframe, cracked brick or bad pointing? a wet brick can extend the drying time a lot.

possibly the plaster was over-polished with the trowel, which prevents paint sticking. You can roughen the surface slightly with wire wool or 150 grade sandpaper, after removing the loose paint. Sponge off any plaster dust or powdery film. Dilute the first coat of matt emulsion with 20% water to help it soak in.

Never use PVA on plaster you are going to paint.
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Yes I agree either the ingress of moisture or polished skim, if I had to choose I would say the latter.

I think you would be fine following the advice given by JohnD here, I if I may will point out possible flaw, the use of wirewool when useing waterbased paints can become problematic regarding its longevity. The use of wirewool will leave metal deposits on the wall and the application of a waterbased paint will cause these to rust, these could and I do mean could in time appear on the surface.

ah yes, forgot that, rust marks, thanks.
No probs John, to be honest this is a rare occurance, the metal particles will become trapped within the first coat and sealed within the second.

However it remains a possibility.

Thanks for your suggestions...
The walls are both internal.. so no possibility of water pipes/dampness...
I will sand down and try again...

Thanks once again.

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