New plaster walls, sealed, but paint cracking & lifting off. Any ideas?

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the plaster was completely dry before sealing with a proprietory sealant & when that ran out with pva diluted as per instructions. Put on a thin base coat of paint which has ...show more
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  • Taf answered 6 years ago
You been listening to people who don't know what they're talking about.

Under normal circumstances you do not seal, PVA or use specialist primer on new plaster you are going to paint. You simply use three coats of water-based emulsion - the first coat thinned (mist) to soak in a little and grab the plaster, that is the primer. I and other 'qualified' decorators have been taught that way in college and been doing it that way for decades - I also worked as the decorating advisor in a decorators merchants.

When you use PVA or another sealer you stop the emulsion primer being able to soak in. If you use any oil-based sealer you suffocate the p[aster. If your walls were rendered and skimmed then you use an emulsion that has no vinyl to allow the walls to continue drying out (can take 12-18 months) after this or if your walls just had a skim coat, then vinyl matt is fine.

If you want to do something useful before painting new plaster wash it down with clean water to get rid of any powdery residue that can act as a barrier between your paint and the plaster and if you have time brush your first coat of thinned primer emulsion on, working it in to get under any powdery residue.

In very exceptional circumstances, like a plasterer has over polished the finish or you don't want to take any chances, then you can apply a specialist primer like Bullseye 123 which is expensive and will adhere to the plaster.

Without seeing your walls I can't tell how far you have to go with rectification. If its patches then use a palm sander with P100 grit (or elbow grease with P120) to get back to what feels like a solid surface and wash any powdery residue off. Then recoat with emulsion. Allow plenty of time now between coats as you already have lots still hardening underneath. Good luck.

Source:

Qualified Decorator - nearly 30 years

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5 out of 5
Great answer, I always used to just use diluted paint first but was told I was doing it all wrong!
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  • ulmets answered 6 years ago
    Assuming that your plaster walls were fully cured (not just dry) before painting: The problem is that you don't seal plaster unless you don't plan to paint. If moisture is a serious issue for you and you want to, you can seal it and then add a plaster skim, then paint that, but not directly paint the sealer. You do NOT NEED TO SEAL plaster walls though. Professional plasterers use a bonding agent between layers of plaster which is just a less diluted PVA usually -- as you know, it is a sealer as well as bonding agent, so moisture shouldn't be a problem except in old walls where the materials are simply old and failing.... PVA slows down the curing process of plaster enormously (because it's a sealer and prevents moisture from leaving the plaster), which is why it can take so long for plaster walls to cure. I don't know how or why the idea of sealing right before painting is getting passed around, but it is. It is a bad bad idea and, while I've heard of a few people doing it and having it be okay, I've heard many more cases of failing paint just like this.

    The paint has trouble gripping sealer and so it runs off (patchy or beading paint) or flakes off later. The cracking paint is early stages of flaking (new paint somewhat lifted the base coat off of whatever grip it had found on the sealer -- it will fail eventually). To fix this, the paint just needs a surface it can grip.

    You need to either 1) get your walls entirely scraped of loose paint, entirely sanded, cleaned up thoroughly, put a bonding agent over that (diluted PVA, for ex), and then skim over in plaster, let it cure, then prime with diluted emulsion paint (a couple of times), and then really paint over that, OR 2) scrape/sand the entire walls smooth and flake-free, clean up thoroughly, and just prime/paint over that, accepting that the walls will only be as smooth as you sanded them (which will not be perfectly smooth). Make sure that corners and edges get as much attention in the sanding as the rest of the wall.
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  • Amazingwoo answered 6 years ago
    Even though the plaster was completely dry you shouldn't put anything on it - that includes sealants - for several weeks in order for it to fully dry out.

    The paint is peeling as there is probably some moisture still deep in the plaster which is slowly making its way out.

    Speak to the builder who did the plastering for advice on what to do.
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  • navi answered 6 years ago
    Hi, problem is normally lime coming out of the plaster. This has to be neutralised. This can be done by washing it with a strong water,vinegar mix. Scrape flakes off, wash with mix, rewash with mix then emulsion after gently treating edges of flakey area. You can PVA before repainting.
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  • RICHARD X answered 6 years ago
    I would have give it a mis-coat first (watered down emulsion), before using the sealant.
    Sand the flaking areas and do as above.
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  • nendlin answered 6 years ago
    give it a wash coat first water mixed with emulsion.
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  • Bilbo answered 6 years ago
    Normal spec calls for a 'mist coat'
    - refer paint manufacturer's recommendations.
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