Old Paint Peeling in Sheets and Bubbles in Plaster


Postby david1974 » Wed Dec 06, 2017 6:25 pm

Hi.

I recently moved to a 1955 house and tried to sort out the ceiling in the bedroom. It looked pretty bad but was sound. I sanded it down and removed all the loose paint, and decided to give it a skim of plaster. I painted it with PVA and when I came back there was a few bubbles appearing so I scraped off all the loose paint, some parts came off easily back to the, I assume, original grey plaster. I re-PVA'd the ceiling and skimmed with a thin coat of plaster. Came back and there were more bubbles. Scaped off every loose bit and gave another skim, more bubbles appeared. Every time the ceiling gets wet there seems to appear a different patch of loose paint. When I try to get it all off I can't, I even tried a wall paper stripper. Should I use a chemical stripper or is there something that I can paint on that will penetrate the old paint and fix it to the old surface better?

Please help

David
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Postby DaveyTed » Thu Mar 22, 2018 7:27 pm

If you used PVA to seal the surface before painting it stops the plaster skim drying out correctly.
You can use PVA as a bond before skimming but not under paint.
New plaster should be sealed with thinned contract emulsion which allows plaster to breath. I have found an excellent alternative to thinning is to use Screwfix emulsion for new plaster. Use unthinned and it bonds well and covers easily in two coats in my experience.
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Postby Nick-Horn » Mon Mar 26, 2018 9:35 pm

Hi there.
This is a common problem I here quite often.
I'm new to paint but it seems a lot of people use PVA under paint thinking that it will increase adhesion or seal the substrate.
I'm told by my chemists that this is bad!
We produce specialist water based product so I shouldnt say this is how it is for every ones products.
Basically in paint the binding system is developed specially for a specific type of adhesion and substrate. If you put a poor quality resin like PVA on first the new coating will bond to that. The PVA will not be able to cope and peel off taking your paint off with it. Or you will wet the PVA again and it will cause issues when applying your top coat.
Also (I know every one does it and it works so don't shout at me) we would not recommend using a cheap emulsion as a mist coat so seal the plaster. Although it's much better than PVA it can have a similar effect.
In addition to using cheap ingredients to bond they can use cheap pigments these will be chalky and over time the chalk used will loose it's bond and take the top coat off with it (if you wipe your finger over some of these products you will see a residue, this is the type of thing I mean)
Also using multiple coatings from different systems can have adverse effects.
If the plaster is not fully dry or the building has some movement in it the different coatings will flex at different rates and this puts more stress on the dry films. This could cause durability issues..

Ideally you need good penetration into the plaster using the same type of product that you would use for your top coat.

Hope this helps a little.
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Postby david1974 » Tue Mar 27, 2018 9:26 am

Thanks for all your advice. I gave up in the end and boarded over the 60 year old paint. I spent way too much time on it. I wish I had done that from the start!
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