Lime plaster has different properties from modern gypsum walls. Lime is a breathable substrate which regulates moisture, allowing it to escape from the fabric of the building. This natural process reduces damp, mould and structural damage related to trapped moisture.
Lime plaster doesn’t always require priming, but if a primer coat is required it should be an appropriately breathable paint. Using the right primer allows the lime plaster to function as intended, allowing moisture to pass through.
Why is Lime Plaster Breathable?
Lime plaster is a natural, porous material which is made from slaked lime, sand, and water. This natural material is highly breathable, it lets moisture pass through freely and it’s naturally high alkalinity deters mould growth.
It is a highly flexible material, which means it copes well with natural building movement. This makes it particularly suitable for use on timber framed buildings where gypsum and cement based plasters would suffer cracks and damage.
Breathable Primers
Most fresh applications of lime plaster won’t need a primer, a mist coat of breathable paint will normally suffice. However, if you are applying a primer ensure it’s breathable (also known as vapour open).
Grabbing a tin of paint from the local DIY store will compromise the plaster. Standard modern primers usually contain PVA or acrylics which will form a seal on the surface. Used on lime walls, this can create real problems. Moisture will be trapped inside the wall, which can cause bubbling, flaking, and even damp patches which will damage the plaster.
An appropriate breathable primer paint will stabilise the plaster surface without compromising breathability. It will soak in and bind any loose particles, allowing paint to adhere to the surface smoothly.
When Does Lime Plaster Need Priming?
Most freshly plastered lime walls are suitable for painting without primer. Simply apply a mist coat to ensure good adhesion and smooth top coats.
However, lime plaster will benefit from priming if the plaster has a patchy texture, or has been repaired with lime filler. Some older lime plaster may feel dusty, or have been repaired with gypsum based plaster in the past. In these situations, a breathable primer can help level out the absorption so the topcoat of paint dries evenly and creates a strong bond.
What to Look for in a Breathable Primer
Make sure you consult the paint data sheet before application. The word breathable is not regulated within the paint industry, which means some products that are not sufficiently vapour open will be labelled in this way.
Genuinely breathable primers will be water based, and free from plastic based binders and solvents. They’ll be “mineral compatible” and have a low Sd value. Anything above 0.5 is not suitable for application on lime walls.
Look for the words open pored, vapour permeable, and breathable.
Auro 301 Natural Plaster Primer - Our Top Recommendation
We’re big Auro fans here at Green Shop Paints, and we think the best breathable primer for lime plaster is Auro 301.
It’s compatible with mineral surfaces such as lime and clay. It’s also suitable for application on gypsum and fibreboard, creating a consistent base for natural paints while maintaining full breathability.
Auro 301 is a water thinnable, solvent free formula that uses the company’s innovative Replebin® binder. This is a renewable, plant-based resin that delivers high performance without synthetic additives.
With an open pored structure (Sd < 0.1 m) and VOC content of less than 1 g/l, it’s an environmentally sound choice for heritage restoration, eco builds and healthy homes alike.
How to Apply
To use, simply dilute it 1:1 with water, stir well and apply with a ceiling brush or roller. It penetrates the surface evenly, binding dust and evening out the surface. Once dry, typically after four to six hours at room temperature, it leaves a clear, breathable base ready for painting. It’s compatible with all Auro wall paints and other natural or mineral paints, and its transparent finish won’t alter the wall’s texture or colour.
Using Lime Plaster Primer as a Sealant
If you prefer a natural look rather than painting over lime plaster, Auro 301 can be used as a clear breathable seal. Just mix two parts primer to one part water for a subtle breathable finish. When dry it deepens the tone of the plaster, enhancing the plaster's natural characteristics.
Alternatives to Priming - Mist Coats
If your project doesn’t require a primer coat, a mist coat is very easy to apply. Just thin your first coat of paint by around 20% using water. This will help it soak into the plaster, creating an even and stable surface for subsequent coats.
Suitable Breathable Paints for Lime Plaster Include:-
All of these paints can be applied directly to lime plaster surfaces without a primer, provided the wall is clean, dry and free from dust.
How to Prepare Lime Plaster for Priming
Before applying any primer or paint, make sure the surface is sound.
Brush away loose dust and debris, remove any greasy marks and fill cracks or holes with a compatible lime based filler such as Auro Natural Wall Filler No. 329.
Read more: Breathable paint for lime plaster | Best paint for lime render
Breathable is Best!
The best breathable primer for lime plaster will support the substrate's natural performance without creating a seal that traps moisture. We recommend Auro 301 for your project.
If you need any further advice on decorating your lime plaster walls, please contact us. You can also order a free colour chart if you’d like to explore our range of colours suitable for breathable walls.
