Our seed bank is building a small prototype earthbag house for the purpose of experimenting with such structures for modifying temps for more ideal seed storage for places off the grid. James, one of our interns, has done a great job in researching and constructing the house. He's just about to the point of applying a lime plaster to the exterior but needs to know about the appropriate type of lime to use. James can correct me, but I think we're looking for some like quick lime. I guess we need to know whether such lime is absolutely necessary or if something else (and locally available) is suitable. We're hoping you might enlighten us whether quick lime is available in these parts or whether we should look at alternatives (names in Thai and possible sources would be helpful too).
Protecting an earth house against the elements is a good idea. I think the most important thing is to have a good anti-capillary layer to prevent the wall from sucking up moisture from the ground. You're beyond that stage though.
There are many ways to render an earth building. I can't say that I've worked with lime myself but I will share with you what I know. Quick lime gives off heat when mixed with water; working with it is a little dangerous. Hydrated lime is lime that has already had water mixed with it. I think that you can get this in dry form but I could be mistaken. Hydrated lime is also available (in rural China) as lime putty; a bit gooey. Probably the safest and nicest form of lime for what you're doing is white wash. Unfortunately I've never seen this in Thailand. If you find it, let me know. Lime in Thailand has 2 main uses: for marking football fields and for processing sugar (I can't remember which type they use). It is rarely available in 50kg bags like cement in building material stores. One reason for this is is that cement is cheap in Thailand. In most places it is probably cheaper than lime. Thailand has a "mixed cement" available which is a high lime content cement specifically formulated for plastering and mortaring. It sets slowly.
For your application, although white lime would be more attractive, it is probably more practical to just use a high-lime cement like TPI green, or Tiger brand cement.