Advice on painting salty walls!!

[marandind]

We live in a 200 year old cottage by the sea, which has flooded once or twice over the years, and now has very heavily salt-impregnated walls. If we paint the walls (emulsion), the paint blisters off (fairly rapidly) over the areas that have been previously immersed.

This year we had the walls stripped and replastered, but the salt still comes through the new plaster, and “ejects” the paint!

If anyone has any suggestions of a “barrier” coating or a suitable coating that we could apply that would prevent this from happening, we would be very grateful. It’s just a little messy, as you can imagine.

Many thanks

 

[emilybronte]

My OH (who is a chemist) tells me that the salt is being pushed out by water still in the walls and until they dry out, there is not much you can do (other than dry line the walls, which presumably you don't want to do, in a cottage).

 

[Flyboytim]

Chemically you have the major salt sodium chloride (there are others, but less important)in a wall that is recently replastered with calcium sulphate dihydrate - gypsum plaster - I guess. In the presence of water, the only very slightly soluble calcium sulphate and highly soluble sodium chloride "do a swap" to give a soluble calcium chloride, and a soluble sodium sulphate. Calcium chloride is hygroscopic and deliquescent. This means it absorbs moisture from the atmosphere and then the salt dissolves in the water that it has absorbed.

So, firstly, the gypsum plaster is being degraded to a soluble, damp mixture of salts.

Generally, building materials like cement and plaster tend to be alkaline - which means they will react with acids that they come into contact with. One such acid which is found as 400 ppm in the atmosphere is carbon dioxide. This can replace the soluble calcium salts with a much less soluble calcium carbonate. The net effect is that the soluble chloride and sulphate ions can get diluted in the matrix of the plaster, and the carbonates build up near the exposed surfaces.

The sodium and calcium carbonates can together or individually cause surface efflorescence.

The original plaster on the cottage was probably lime and sand - the traditional mix which naturally matures to a porous calcium carbonate matrix, which is less likely to react with salt water, and rather than paint has a traditional lime wash finish, which is never likely to be lifted by efflorescent salts. Indeed, local materials would have been used, and the cottage was probably built with some materials sourced from local beaches with a high salt content.

Lime wash can be tinted with modern, as well as traditional pigments to achieve the right decorative effect on a matt surface.

So the suggestion is to use traditional building materials and wall decoration.