Making Your Cob Walls Smooth before the Final Render

So once you have built the structure of your cob castle, you are probably going to want to make the walls smooth so that you can effectively apply your final, more water resistent render. I have written a post already about how we made our water resistent render using cow poo.

I am writing this post during a two week programme we have made here at Eco Caminhos, Brasil. We have a group of scouts visiting from Belgium and we saved some spaces for them to try out our smoothing mix. We call it the Margarida mix, as one of our volunteers called Margarida, came up with it. So you want to get your walls nice and smooth, free of cracks, holes and large bumps. Your proportions may vary due to the sand and clay content of your earth, so it is good to do your own tests before making this mix. But as a starting point, our mix is made of the following proportions…

  • 1 sand (filtered to about 3mm)
  • 1 clay (again filtered to 3mm)
  • Usually about half a bucket of short-cut straw
  • Add water till it is a workable mix.


First of all, as seen below, you are going to need to filter your clay and sand. You cannot have large stones in this mix as it will make it very difficult to apply the mix to the wall. The finer you filter, the smoother your mix will be. However you dont want this mix to be perfectly smooth as you are going probably need to do one final render with a waterproof coating.


Next, pour your sand and clay on to the space you are going to mix it. Some people use a wheelbarrow, or bath, or other suitable container, we are using tarpaulins. You can lift the edges of the tarps to throw everything back in to the middle.


Mix well with your hands or feet, or by pulling the ends of the tarps to fold and roll everything about.


Slowly add water and straw. Too much water is always more difficult to rectify than too little so better to go slow.


Once your mix is ready, put it within easy reach of the space you are working on. To apply you need to wet the wall with a large brush. WIth the brush, make straight strokes in the same direction until you see a shiney layer of clay on the surface of the wall. This is not done by adding more and more water but really by adding a bit, and then brushing till you see the shiney layer. Then take a bit of clay slip (fine filtered clay mixed with water in to a nice paste) and brush the slip over the wet space you have created.


Once the space is wet and slipped you can apply your Margarida Mix by hand. I find it is best to take a small amount that easily fits in to the palm of the hand, and work it in to the wall in an upwards movement. This method stops you from dropping too much on the ground.

It is also good practice to place some boards or tarps on the ground below where you are working to catch all the mix you drop. This will save you time making new mixes.


Any questions, please submit them below, or via the contact page.

Good Luck!

5 Comments Add yours

  1. Trevor Keeling says:

    Really interesting Tom. Keep on posting


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