Yes, you can do it for no cost if you're willing to scavenge the materials yourself. A good plaster can be made from clay, sand and a coarser gravel. There are many recipes and lots of add ins but many of those are free as well like cattails or manures. I built myself a straw bale house and have used my site clay and local sand and gravel for most of the plastering. I did use a straw/clay mixture for the scratch coat but if I hadn't had straw I could have done it without it.
Yes, there is a 100% DIY, 100% free natural plaster. It is called earthen plaster. You mix it from your subsoil. In Colorado where I lived for 36 years, I used my subsoil, screened out rocks, added water, and plastered my walls with it. In most places I've worked, though, I find that the natural subsoil has too much clay and I have to mix one part subsoil with two parts sand, which I have to buy...So, unless you can find a free supply of fine sand, you will have to probably spend a little money to make a natural plaster.
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