Paint Turns to Powder

James Hay Staff asked 3 years ago

I am renovating an old cottage with original walls built with stones and mud with a cement finish inside and out. My problem is the interior paint turns to powder after a few months and falls off the wall, this happens on the lower part of some walls and only in some areas. We checked for damp, water ingress etc. and it happens on walls to the outside and interior dividing walls.

The cycle is paint with an interior matt water-base white paint, after a few days a few surface bubbles, but not everywhere, and then later the paint “grows” white dry fur of between 2 to 5 mm high and turns to powder so you can brush it off the walls. Sealers, undercoats etc. do not help much, they only delay it by a few days and I am beginning to suspect a chemical reaction from the walls.

There is no damp or any kind of mold on the walls, it just seems to react and dry the paint to powder. Acrylic white paint does the same thing!
Any advice or help would be appreciated.

3 Answers
crowderpainting Staff answered 8 years ago

My best guess is the cement finish has degraded a bit and has a very high PH. The high PH will "burn" primers and sealers, plus cause adhesion problems (bubbling and peeling). The white fur you described is like a efflorescence deposit that is forming a weak crystal.

One way to test this theory is to collect some of the powder and use a soil PH test kit. Proceed with the repair if the PH is off the chart.

The only way to deal with this problem is to remove the paint from the affected walls, wash the cement finish with a mild acid, white vinegar and water (1 to 4 ratio), and seal the walls with a stucco primer/glue. The sealer is used by stucco contractors to seal an area before any repairs. This isn't a typical primer available at a paint store. Call your local stucco-plaster supply houses for availability. An alternative sealer is a pigmented shellac, not as good but it might work.

Test this approach on one of the affected areas before proceeding with all the walls. Scrape back the paint from the area and expose the original cement. Wash the area and allow it to dry then apply the sealer. After the sealer has dried, primer the area and paint.

Anonymous Staff answered 8 years ago

Thanks for the advice, I had been leaning towards the chemical reaction from the wall surface but I am not sure what the current PH is but will use your idea.

After digging around Google for some time eventually found the average PH for water based paints which are already alkaline so the theory fits to a severe reaction breaking down the paint structure. I had planned to try to establish the PH of the wall and then give a test piece of the wall a mild wash treatment of the opposite PH to try to bring it back to as near neutral as I could before trying the paint again.

Failing that I might just follow the advice of our local paint shop and strip it back to the old cement finish and just white wash an area to see if that sets better on the old walls.

Anonymous Staff answered 8 years ago

Prevent peeling off paint due to salt content. How to prevent peeling off paint due to salt content in the plaster or brick.

Many people face this problem in their interiors. I know one standard procedure as I am in the construction chemical field and what I suggest is;

1. First remove the plaster till brick masonry.
2. Do Acid Wash of the wall till you find white spots coming out. Every time this wash should be followed by mineral water wash to remove the white spots.
3. Then apply one or two coat of Acrylic Binder and then go for painting.

Now after doing this also one can't be sure of not having problem in future. I want to know that whether any paint in the market is available which can withstand to this.

We have used sand like paint before few years in our old house which was very good for it but it was costing around 25 Rs. per Sq.Ft. Please suggest me the solution as soon as possible.

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