Plaster for mass heater

Hey everyone, I have been working away building a rocket mass heater. It is mostly all done except for the outer plaster. It's a surface that will need to be durable. I wanted to mix a harder plaster clay/ sand & lime. I read somewhere that the lime will make it harder. But I don't know what are the best ratios and if lime really does make it more durable. Everywhere I look seems to have different information about plasters. I'm Looking for advice and suggestions. Thanks you!
Christine

Hi Christine; If you have just finished your rmh , you will want to let it dry for at least 2 months to make sure all of your moisture is out. If you seal coat to soon you will be sealing that moisture in. Sorry but I don't have a mixture for finishing but I'm sure somebody else will have a good mix for you.

Thanks Thomas,
I was thinking that mixture still allowed for evaporation. But I've never done this before and don't really know. I was planning on sealing it after it was completely dry with a mixture of linseed oil and beeswax. All advice is welcome!

i also would like some suggestions with formulae for rmh plaster. thanks.