I've not done it but know someone who done that, he used some kind of weird clay manure mix on the
wood first that functions as a glue between the render and the wood. I asked, he replied as well. He says he made it of fresh cow dung mixed in clay. Leave it for a night, then add a flour glue mix and add sand. A liquid sandpaper en put it on the wood with a paint brush. Render all. If it cracks the wood is too wide, he added a strip of jute sackcloth then, especially where wood borders straw. The sand acts as grip for the next layer. The man and his wife done a whole house. Good luck, hope this helps.