Easy to understand/mix/find plaster ideas for my RMH

I am just about to finish construction of my RMH and I am trying to look into the best option for plastering. (Best being easiest to do with easy to find supplies.) It is in my greenhouse so it will need to resist moisture fairly well.
Also, I'm assuming that everything is going to have to be very dry prior to being plastered....so it may be a while before I can do that part. This is my first RMH and I've never mixed a plaster before.

Hello Amy,   I don't have any experience with building a rmh for a greenhouse, but I do know that Erica and Ernie have plans available which covers a lot about a greenhouse build:   Greenhouse-Rocket-Mass-Heater-Plans

Lime plaster comes to mind or using tiles. Hopefully someone with more experience in this are can help out. It'll be a learning lesson for me too :)