Test and Apply a Natural Finish for Clay Block Wall





I am currently working together with Eco Caminhos  Brazil on a social project to test and apply a natural finish for the exterior of a local mans house. The man is 90 years old and as fit as any 40 year old. For years he has dreamed of finishing his house with a final render but has never been able to.

We managed to make a strong mix using lime as the binder, so if you would like to see the process of testing through to final application, please read on.

Thanks to everyone who offered advice on this project, Cobi, Tomek again and the helpful face bookers over at BIOconstrução Saudável and BioConstrução

A lot of the advice recommended using one part cement so we did some tests with both cal and cement. The test mixes and results are at the bottom of the blog here.

So first up you’re going to need a wall to test your mixes on. The wall we are going to render is made using these concrete blocks, stacked with concrete between them. I tried to recreate the wall as best as possible. I stacked the dirt up behind it to make sure it was strong when I pushed against it with the renders.
Get all your kit together first, it will make things easier when you get going. First of all I used the filter to sieve the sand and clay to the particle size we wanted (about 3mm)
…then the rest of the kit:
  • Milk boxes to make the mixes, numbered to show which mix they contain
  • A bucket of sand, clay, cement and lime
  • A measuring cup which represents one unit (pictured here in my hand)
  • Something to stir your mixes
  • A hose or bucket of water
  • Spatula or hock
Taking ‘one part’ lime
Add all your ingredients to the mixing pot and mix well until you get a consistent render
Make sure to wet the wall first with water and a brush. If it very hot you will need to wet right before application as the water will dry quickly. I used a spatula to apply the mixes as I didn’t have much space. I think the bigger the better with your test mixes so go as big as you want. Make sure to number your mixes so you know what quantities are on your test patch

…And then you wait. The longer the better really. You want to know how the mixes react to the various weather conditions they are going to be experiencing. Luckily the weather is very changeable here so we never really have to wait longer than a week to get a good range of heat and cold, rain and sun.

Make sure to keep the lime mixes covered from direct sun in the first few days after application. You should also wet them regularly to ensure they do not dry too fast and crack.

Our Test Mixes & Results

  1. 4 sand : 2 earth (type 1) : 1 cement
  2. 6 sand : 3 earth (type 1) : 1 cement
  3. 3 sand : 1 lime
  4. 2 sand : 1 earth (type 1) : 1 lime
  5. 2 sand : 1 earth (type 2) : 1 lime
  6. 2 sand : 2 earth (type 2) : 1 lime
  7. Cow poo mix

Application notes

  1. Easy enough to apply, a bit gritty
  2. Easy to apply but again a bit gritty
  3. Quite difficult to apply due to high sand content.
  4. Easiest to apply so far
  5. Easy to apply
  6. Easy to apply
  7. Easy to apply

All mixes stuck to the wall without too much problem. They all needed a bit of a push to stick nicely.

And then we went to a guys house to render the whole place in one weekend. Here are some photos of the event, with a description below describing the technique or what is happening.


First of all we sieve all the ingredients. Here are some happy scouts sieving the sand and clay.


Above is a mix consisting of 2 buckets of lime, 2 buckets of earth, and 4 buckets of sand.


Mix them all together on a flat piece of earth and when you have a homogenous mix, start adding water till you reach a consistency of a nice paste.


Whilst you are making the mix for the render you can fill any large holes in the wall. We used a Margarida Mix which we are familiar with using on our cob walls.


Start by throughly wetting your wall, and make sure it stays wet up until you apply your mix. The render probably will not stick if the wall is not wet.


Then take a big lump of render and really push it in to the wall, in an upward motion to keep the mix from falling to the ground.


Here was our pro for the day. Expert renderer.


Afterwards we used a sponge pad to firmly wipe the walls down. It gives a slightly rough but even finish. You can also use it to merge in the line between a section of dry wall from the previous day, and a piece of wall you have just done. Really push and rub the sponge in to create an even finish.


And here is the colour of our beautiful natural lime finish. Ready for a good Limewash Paint, or good in its natural pale colour too.



General Notes.

  • Type 1 earth is clay that I found at our property. After I had made tests 1 to 4 my boss told me we had a bucket of clay from the site we would be working at so I made some extra mixes with the new clay (type 2).  
  • I tried our cow poo mix to see if it would stick to clay bricks, just as a bit of extra knowledge. I know the mix doesn’t last for a very long time under direct rain and would need to be reapplied and reworked each few years, as a result we cannot use it at this project as the finish needs to be tough as we may not be able to go back and reapply in a few years. 
  • The mixes which contained lime did not need any added water to make them in to a plaster, the mixes with cement were dry powders so I had to add water. 

Advice Received

  • “I will only add that clay and cement are not good combination, in both natural and industrial civin engineering when you have a lot of clay you add lime not cement, clay and lime work good together. Also you cant put cement plaster on clay brick wall for another reason-it makes wapour barrier for water and mosture travelling tjrough the wall. It gets captured between plaster and the wall, after some time it causes serious damage like separatin the whole plaster or even damaging the wall…”
  • You can add the following to increase water resistance: PVA glue, cactus ‘juice’, linseed oil, cow poo
  • “You can use a mix of 3 sand, 1 lime, or maybe add one earth instead of a sand. Make sure the sand is coarse well graded. Wet the wall first. Keep the mixes protected from the sun in the first few days and wet them regularly to stop them from drying to quickly and cracking”





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