We have now refined the process. First up, I have decided to abandon using the render gun to apply a 10mm coat of 1:3 clay to sand mix in the first pass. The reason for this is that, as good as the gun is, the sand I am using has too many rogue 10mm stones in it, which block up the gun and the time I was saving was used up by frequently having to unblock it. If you have a better-graded (but therefore more expensive) mix this would not be an issue. Instead I am using the gun to put a clay slip-coat on – reverting to the technique recommended in the holy book. Putting this coat on with a gun is very fast.
We have also experimented with the mesh, working this into the top of the first / main, 20mm coat. Once this is on it is possible to ‘move’ the render around underneath it and flatten things out because the mesh stablises the surface. However, having discussed this with Chris from Back to Earth he recommends not putting the mesh onto the first coat, but rather allowing this to dry somewhat and then put the mesh into a 10 – 15mm top coat. We will try this from now on. I have also decided to add straw into the first coat, despite not necessarily needing this with the mesh, on a belt-and-braces approach.
I have also worked out a good way to get hessian to stick onto the wooden window sub-frames – namely to make a glue from flour, clay and water.
Finally I have made up a batch of ‘sticky straw’ to build-out around the windows. Since this involves creating a layer at least 50mm thick, this is too much for a render layer – even with lots of straw in it. Instead this is more like a very straw-rich cob. I used a mix of around 1 part flour, 2 parts clay and 2 parts sand – plus quite a lot of water and sufficient straw to get a mix to a point where the straw readily sticks together and forms a sticky, maleable material.