How Do You Stop Salt Coming Through Newly Plastered Walls?

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How do you stop salt coming through newly plastered walls? I used limelight renovation plaster on old brickwork but the salt is coming through the paint just flacks off?

Thanks, Richard Taylor

 

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Hello Richard

The problem of mineral salts can, as it seems you are finding, plague some interior finishes. There are a couple of possibilities: -

1. The paint you are using is impermeable and not allowing the plaster to dry out adequately. So as the moisture is trying to escape the paint is holding it in - Something like Dulux Super matt would be a good paint to use as it will allow the wall to breath and dry naturally.

2. If you have covered that base, the problem is obviously a little more serious. You may have to think along the lines of removing the plaster back to the brickwork, and give it a good wire brush and even rake back the mortar joints 10mm, removing all loose material
- To provide a key for application of a 1:3 cement sand render using a proprietary salt retardant additive which I am sure Luke would stock / recommend (or follow the mix requirements on the additive bottle if different)
- Use washed, sharp sand, and apply to a thickness of 12mm. Again you may have to modify this if you are trying to feather into existing plaster. This will be a scratch coat to receive a final finishing coat of plaster. Use the minimum of water, and trowel the render mix well into the raked joints, but don't over trowel.

3.If the hygroscopic salts are really plentiful, you may have to consider a partial tanking exercise using something like RIW and keying it while still wet with sharp sand to receive your render / plaster.

Making these comments at a distance is obviously not perfect, and will depend on factors like your wall remaining damp from other external factors - e.g. driving rain possibly penetrating through the wall and keeping the salts active.

So before you start ensure you get the wall as dry as it can be before re-plastering and let the new application dry out thoroughly before decorating, - even then make sure you use a vapour permeable paint like super-matt.

A bit long winded, but I hope that something was useful.

Mark
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