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How to fix damp plaster on internal wall?

My flat has heritage sandstone block walls which on the interior have been rendered over and then plastered. There is rising damp up to about 1 metre from the floor level in many areas, especially on a dining room wall and in bedrooms. Due to the sandstone there is no possibility of a damp course being inserted and injected damp courses have failed too.

Is there a paint - possibly lime-based - that I can use to work with the damp rather than enclose and fight it? The previous acrylic paint has done a lot of work but finally failed, flaking off in large pieces exposing damp plaster while other areas are worse with very wet damp mouldy patches.

I have stripped off the flakes in the dining room, exposing slightly damp plaster. This is where I think a special paint type might work well.

The bedrooms walls are very bad and I have not tackled these yet.

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EricL

Hello @shmorgan13

Welcome to the Bunnings Workshop community. It's sensational to have you join us, and thanks for sharing your question about your sandstone wall.

I've just been in contact with Crommelin technical, and they suggest looking at using Crommelin 2L Dampstop Concrete And Masonry Damp Surface Moisture Barrier And Primer. They recommended using Crommelin 1L Waterproofing Membrane Surface Primer on the wall surface first before using the Crommelin Dampstop.

Their general suggestion is to clean your wall of dust and debris, but should there be evidence of mould and mildew to use a proper cleaner to clean the wall. They have also suggested to use a heat gun on the wall to remove any surface dampness before applying the waterproofing membrane and Dampstop.

This would be the only substance that I'm currently aware of that will withstand both positive and negative hydrostatic pressure.

If you need further assistance, please let us know.

Eric

shmorgan13

Thanks for your prompt reply Eric.

I am reluctant to use something that "contains" the damp under any paint, which is what seems to be suggested by Crommelin, as the damp would then have nowhere to go. We have tried lining walls with waterproof sheeting (a builder's idea) and this has made the damp even worse, doing goodness knows what to the plaster wall still underneath and forcing the damp into floor/carpet and ceiling.

What I am thinking of is a breathable paint which allows moisture to pass and dry out with proper air ventilation in the rooms.

EricL

Hello @shmorgan13

The only product that I'm aware that works the way you describe it is the Dulux Trade Vinyl Matt Emulsion. But before you go looking for that product, I suggest speaking to a painting professional first and asking them if it is applicable to your current wall conditions. This product is only available at the Dulux trade centers and can only be applied by a painting professional as it is a trade product.

My only other suggestion is to consult a damp & waterproofing specialist who is familiar with condensation issues.

If you need further assistance, please let us know.

Eric

MitchellMc

Hi @shmorgan13,

You're absolutely right to be cautious about using products that trap moisture. Sealing damp into a wall, especially with non-breathable paints or membranes, can often make matters worse over time — just as you've seen with the waterproof sheeting. That kind of barrier tends to push the moisture elsewhere, like into floors, ceilings, or other adjacent materials, rather than allowing the wall to dry out naturally.

Traditionally, walls that needed to breathe were treated with coloured limewash. Limewash is a highly breathable finish that not only allows moisture to pass through but also has natural antibacterial and antifungal properties. It was commonly used for exactly this reason, especially in older or solid-wall constructions without modern damp-proof courses.

While I’m not aware of any modern paint that matches limewash in terms of breathability, there may be some niche products or pigmented mineral-based washes that come close. I would strongly recommend speaking with a specialist in historic building restoration. These professionals often have experience with exactly the sort of breathable finishes you’re looking for and may be able to suggest materials or methods that aren't widely known in general building practice.

It’s definitely a bit of a niche area, but one where expert guidance can make a big difference — both in preserving the structure and avoiding ongoing damp issues.

Please let me know if you have any questions.

Mitchell

shmorgan13

Thanks Mitchell. I think you are right and will contact Heritage NSW or similar as it will have been doing this work in similar buildings.

MikeNYC

Exactly the same problem. In the middle of repairing it myself. Working on adding 1/4 cement boards to plaster walls to get a more consistent surface (3x5 foot, only doing bottom of wall). Using cement screws.
Next step is to add 1/4 x 1.5 inch painted wood strips to cement boards using liquid nail glue, and over that adding 1/4 drywall. ( both sides primed and painted.

I’m hoping the gap of the 1/4 wood strips will let the moisture escape and protect the drywall.

Only downside is that I’ll have a chair rail looking walls which I’m not too fond of

MitchellMc

Welcome to the Bunnings Workshop community @MikeNYC. It's brilliant to have you join us and many thanks for jumping into the question.

Working with older building methods can be tricky. That’s a clever workaround, especially using cement board as your base. The small gap idea could help, but with sandstone you really want to be careful not to trap moisture as those walls need to breathe, both internally and externally. Consider how moisture might escape the cavity you are building. Could you provide an air gap somewhere that allows airflow behind the sheeting? Sealing off the surface and trapping moisture in there is not ideal.

Please let me know if you need any assistance.

Mitchell

MikeNYC

for air flow I left 1/4 gap for both cement boards and drywall from the floor. Still working on an idea for the top of the wall, maybe L shaped trim not flush with drywall.

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