Lime wash – how it’s made and where it can be used…

What is lime wash?
Limewash is a traditional form of paint which has been used for many thousands of years. It can be applied internally and externally to decorate or protect lime plaster or render, or on any brick or masonry pointed with lime mortar. It is highly vapour permeable, so it lets water vapour out of the walls whilst also offering a water repellent shelter coat.

Vapour permeability
‘g/m²/day’ (grams of vapour moisture able to pass through a surface coating measuring 1m2 in 24 hours) is a measurement that truly evidences the incredible difference between ‘breathable paints’ and lime wash.

Standard acrylic masonry paint – 0.7 SD – 50g/m²/day
Microporous acrylic ‘breathable’ masonry paint – 0.5 SD – 180g/m²/day
Limewash – 0.01 SD – 1968g/m²/day

To bring the above data into terms most of us will understand.. beer garden terms that is.. a standard ‘breathable’ masonry paint will let through a double shot every twenty-four-hours, where as lime wash will let through three and a half pints!

Making lime wash
There are three ways of making limewash and they are:

1: Add equal parts of water and hydrated lime (found in most builders merchants) into a bucket, then stir and adjust the mixture until you reach a consistency somewhere between milk and single cream.

2: Take lime putty, place in a bucket, add water and stir until you reach the same consistency as above.

3: A hot lime wash can be produced by slaking (adding water to) quicklime, again, adjusting the consistency as above*.
* please note that this process creates an exothermic reaction and should be attempted with caution.

How to apply lime wash?
– Lime wash is applied with a masonry type paint brush, typically requiring 3-4 thin coats.
– Note – the correct consistency for the limewash should be that when you apply it to masonry you can tell due to the lack of coverage that it will take multiple coats. Applying the wash too thickly will cause it to crack. Coats should be applied 24 hours apart.
– Surfaces must be dust free and flaking materials should be removed.
– Due to the risk of frost damage, we only apply lime wash between the months of April and September.
– Lime wash cannot be applied in the rain and is not rainproof for 48 hours.

What colour is lime wash?
When made with hydrated lime, lime putty and quicklime the natural colour is a brilliant but natural white. Coloured pigments can be used to achieve a huge range of finishes.

Colour Blaze pigments
Cornish Lime pigments

Can lime wash be made more water repellent?
Yes! With the addition of casein, tallow, or linseed oil, however, please note that the addition of any other material with affect the vapour permeability of lime wash. We typically use raw linseed oil (top coat only on exterior lime wash projects) at 1% which equates to 50ml per 5 litres of lime wash.

Link to a scientific study on the effectiveness of linseed oil in lime wash – external link
Link to a scientific study evidencing the SD value of linseed oil paint – external link

How much Limewash do you need?
A 25kg bag of hydrated lime (£18-22) will make enough limewash to cover around 80-120 m2 in 1 coat, dependant on absorption rate of materials.
Materials and cost example – the front façade of a 2up, 2 down cottage 22m2 x 3 coats = 10 kg = £10

Why we would specify lime wash over other types of paint?
Typically we will be presented with a damp internal or external wall that requires the highest degree of evaporation possible at its surface. Not only does lime wash allow for this, it also has a sacrificial nature, meaning that when if it does ‘fail’ and move away from the substrate, it does so in a light, loosely friable manor, moving off of the surface is the form of powder, or lift in the form of easily scrapable flakes. Whereas typical style paints, even highly breathable silicate paints, tent to flake and peel in a manner that leaves behind an unattractive surface that is difficult to sand and repair.