"Anyone have any experience using a masonry sealant on cob? Specifically, I'm interested in cob tub/hot tub applications. I've seen the old video of the cob hot tub attached to the wood fired oven....but this seems to be the only recorded application. I'm sure there are more. I'm curious about long term water exposure and using possibly some portland in the final layer then sealing as you would a concrete hot tub or pool. Benefit would be a "more" natural process and the ability to sculpt the product more than straight concrete would allow. Thoughts? Experience? Photos?"
Cob expert Miguel Elliott (way above my Uncle Mud pay grade) responded when I asked him this exact question years ago:
"Ceramicrete [but it is] hard to find though. Try Grancrete. Have to special order it. Hydraulic lime plaster first, then trowel on the grancrete. That should work. " Above is his video from one of his famous California cob pizza and hot tub party videos.
It has worked for him several times. Great natural builder Massey Burke has also done this with a polished lime plaster called tadelakt which is beautiful and a huge amount of work.
I personally prefer using a metal stock tank or bath tub over a fire or rocket heater. We are working with Becky Bee right now to bring back her classic book on cob hot tubs.