We will be constructing a small 12' diameter round earthbag building in the spring. I am doing as much research as I can, but find two differing opinions at the below websites concerning exterior plaster.
Our original plan for exterior plaster/render was for clay/sand/straw plaster to fill in crevices between the earthbags, and then after curing, add a couple more layers of same plaster, and finally a couple coats of lime plaster to finish off. I read recently that unless you have actual cob walls, or clay plaster thicker than 4", the lime plaster will not stay on because the earthen plaster shrinks and expands and although the lime plaster breathes it does not expand like the earthen plaster, and within a year or two, the lime plaster will crack and crumble away. This same website mentions that you could mix the earthen and lime plasters but should be tested as the mineral content of clay may cause a chemical reaction with lime. Then the other website state that you can lime plaster on top of earthen plaster to weatherproof or make water resistent as the lime allows moisture in and out as it breathes. We do not get much rainfall here, but some snow. We don't want to actually seal an earthen plaster such as using boiled linseed oil over it. We want to allow the earthen plaster to breathe and hence the reason for wanting to coat with lime plaster. We do not want to use any cement or portland cement, at all.
I have read (somewhere) that you can add lime to earthen plaster, but details of if dry lime or lime putty were not given nor the ratios, so I don't know if this is an option or not. Can you really mix lime, dry or putty in with the clay, sand, straw plaster, and then use straight lime plaster for final two coats, without having 4" thickness of earth plaster? I like the idea of using only the earthen plaster, but I don't want to have to maintain it every year and thought with the lime plaster over it would be less maintenance issue, with maybe lime wash every few years or so, but now after reading the one posting, I am afraid to do this as I do not want the plaster to fail.
These are the two websites that concerned me with differing opinions:
I do have experience with exterior earthen plaster and exterior lime plaster. I have a cob house with an attached strawbale walk-in cooler for our farm. The strawbale cooler has cob infill, then earthen plaster, and then lime plaster. It has been mostly done for 2 years, and even though there are some areas that still have only one coat of lime plaster, they are wearing very well. I can see no evidence that the earthen plaster substrate is stressing the lime plaster. The base coat of lime plaster was supposed to be covered with a finish coat all over, but is still only half done, as it has been so patiently showing no wear, that it keeps falling down the priority list. However, I will say that the wall that has the second coat of lime plaster looks so beautiful that I do intend to add the second coat everywhere. Then, the general maintenance plan is to "whitewash" it with thin lime solution every few years. Earthen plaster is generally more forgiving and easier clean-up, but the weatherability of lime was important to me over the strawbale, as I have fear of moisture seeping in during driving rain. I do love plastering, and am now in the process of plastering over my outdoor cob oven. With a good amount of screened horse manure, the earthen finish plaster seems to wear as well as lime. One can always oil it for even more durability. Then it is basically the same as the finish coat of my earthen floor, which baffles me with its durability. However, the lime plaster is literally rock when fully cured in moist conditions, so is even MORE durable than well-prepared earthen/manure plaster.