The photo above shows the final application of elastomeric coating (Maxi-Stretch) to the dome. The manufacturer (Ames Research) pre-colored the polymer to a light tan shade.
I decided to continue to move away from a traditional lime plaster exterior rendering due to the fact that all the natural builders here in Arivaca have had no long-term success with keeping the lime plaster from separating from a cob base wall. This is due to the daily 40-50 degF temperature swingshere, that cause lime plaster to expand and contract at a different rate than the underlying cob wall, thus essentially separting itself from the cob wall.
In the traditional sense, earthen structures need to breathe. Therefore, I plan to use an earthen rendering for the inside of the dome for that purpose. Here in Arizona, the climate is mostly very low humidity, therefore excessive moisture in not an issue compared to other regions.
The coating in the photo above was the intermediate coating of liquid rubber. The trade name is Blue Max. It's really cool to paint-on your own rubber roof. Note that Blue Max is not UV stabilized and requires a top coat of elastomeric such as the applied Maxi-Stretch
I took this photo knowing I would need to show the layers of applied coating to descibe the waterproofing concept for the dome. In the photo above to the right, two applications of Super Primer by Ames Research, which acts as a sticky base coat which hopefully will securely adhere to the straw/clay mix and provide a base surface to build-up the other coating.
Then again as discussed above, I applied to coats of liguid rubber (Blue Max), followed by 3-coats of elastomeric. This brings the weather-proofing to seven total coats for the dome.
Please note that this process of appling a painted-on rubber coating onto cob (straw/clay) is EXPERIMENTAL, with the thinking that it cannot be any more of a maintenance burden that having to replace a lime plaster rendering every two years as has been the case with other adobe building I have constructed.
Also note that these high-tech coating are non-toxic and safe for water-harvesting.