Here is an email I received about plastering straw bale walls today. My answer is below.
We have already done the first two coats and wanted specific instruction on the last as it is the one that will encounter the weather. We have used ordinary clay, gritty ( lots of facets and sizes of grains) sand and chopped straw for the first two coats and they were much drier than what you showed in your video. We also didn’t wet down the bales first, but did sponge the first coat before applying the second. It is very rough so we don’t think we’ll need the scratch thingy.
Our ratio was 3 parts sand to 1 clay and about 1/2 chopped straw. The clay was dry bags purchased through the pottery supplier in town. It was easier to apply with more clay and we tended to put in less sand as the walls went along. unfortunately that meant there are a few cracks now… not really big but there none the less. It seems to be well stuck on and really hard though. It is weathering were there are direct winds and rain, mostly the west side of the house.
So really at this point we are just soooo tired and want someone to come do it and FIX it! We had hoped in the beginning to get someone to use regular stucco for the final coat and thus have a maintenance free exterior. we have heard recently though that the mixtures would react differently to climatic changes and might move or crack relative to each other. What do you think?
I am sorry to hear that you feel you have reached a stopping point. I know how that goes. That said, I think you still have some options for plastering straw bale walls.
If you have a relatively smooth second coat in place, you can fix the cracks with relative ease. If you have the plastering video, go to the scene about floating the brown coat. That is where you use a rubber float to push ad swirl the second coat of plaster into the first. It smooths the surface and strengthens the plaster. If you have already done that, great! If not, you can still do it if you mist down the clay and start floating. This is one joy of clay plasters: you can rework them even after they harden. The same is true for the cracks. You can mist them and float them back into the plaster.
For the final coat, I suggest you use a lime plaster with the color in the plaster. This will allow you to finish the plaster in one shot, no need to paint, and will provide a resistant surface for the elements. I suggest Natural Hydraulic Lime if you can fit it into the budget. You can find them at www.limes.us. That will work over the clay with no problem. It is more flexible than cement based plasters ands therefore can handle the movement. This is a thin layer, so the brown coat will need to be smooth before you can apply the lime finish coat. Good luck and make sure to rest your plaster arm. I have a friend who ruined her shoulder by working too hard for too long on plastering walls.