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PostPosted: Fri Oct 14, 2011 2:38 pm 
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Hi all,

I have bubbling paintwork on a small section of interior wall underneath a sash window. Turns out that the sash was leaking/rotten at the bottom - long story, but the sash is now repaired, so no more water should be getting through.

I assume that the bubbling paint was due to damp, so I have stripped off the old paint in the affected area (roughly 1m by 1m) back to the plaster, and I'm now ready to redecorate. However, I'm not sure what to use as a primer.

The final top coat(s) will be dulux L matt from B&Q - this is from the Dulux Colour Mixing machine, so is retail paint (not trade) and according to Dulux it contains vinyl. I need to do this to match the existing paint on the rest of the wall.

I need to prime the plaster, presumably with a mist coat of matt emulsion (or should I use an oil-based primer?). Can I dilute the dulux L matt and use this as a mist coat? Reading through the forums, it seems that people advise against using "Dulux Rich Matt" as a mist coat - not sure if the paint from the colour mixing range is Rich Matt or not?

I note that many people recommend Dulux Supermatt, a non-vinyl emulsion. But I can only find this is vast 20-odd Litre cans for £40, which seems a bit overkill for a 1m by 1m repair. What else could I use as a primer coat?

I'd appreciate any advice that can be offerred


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PostPosted: Fri Oct 14, 2011 3:04 pm 
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Well the top layer of plaster will be full of salts, and will shun emulsion. Ideally you'd use alkali resisting primer, you might get away with a cheaper thinned oil based primer.

Once this is dry just paint with your coloured Dulux (any).

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PostPosted: Fri Oct 14, 2011 5:48 pm 
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Hi Puma, thanks for taking the time to reply.

You think that efflorescence - salts in the plaster causing the paint to bubble - will still be a problem? I have some Polycell Damp Seal, which is a solvent-based primer for damp plaster walls. Will that be any good?

We have used this before, in the same area, prior to the leak in the sash being discovered. When I stripped the whole area after the sash repair, those sections that had previously been treated with the damp seal peeled off in large sheets - so the damp seal didn't appear to have adhered that well to the underlying plaster. Is this normal for solvent-based primers?

Otherwise, I guess that Dulux Trade Alkali Resisting Primer would be the best bet?


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PostPosted: Fri Oct 14, 2011 8:18 pm 
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Well there is this weird problem which not very many people know about. The water in the emulsion can cause the remaining salts to form a gel which prevents bonding.

You can use the damp seal, sand any weak plaster away, thin the paint out about 50% with white spirit. The idea is to get it to penetrate as far as possible into the plaster - don't try and coat it thickly at all. I would just do this once to keep the plaster slightly breathable (long story). Then paint with emulsion.

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