Plaster powdered is very cheap and its uses in terrain modelling are limitless. It must be mixed with water to produce some sort of paste, which dries quickly becoming rather hard, and this is the only major limitation because you have to be quick to add traces or footsteps before it’s too late. Afterwards, it can be painted very well with any type of paint, from gouache to acrylics, enamels and oils. You can also mix some pigments with it before mixing it with water, to obtain a pre-colored paste (but the final color might change after drying), or you could mix it with colored or even dirty water, go obtain something different from the typical grayish color of dried plaster, like the one below.
If you mix plaster with water and let it dry, you will obtain a rather smooth surface, which might be good to represent “compact” terrains, like snow or sand where no one has passed yet. But if you want to reproduce grainy snow or sand, you can sprinkle plaster powder over the wet mixture of plaster and water before it dries to obtain a convincing effect.
The way you work with plaster it’s only a matter of experience and trial/error: you can lay multiple layers of paste with or without sprinkled powder on it, you can add more or less water in different parts of the layout, you can sprinkle plaster from a teaspoon or let it pass through a tea strainer, depending on the particular effect you want to create. If you sprinkle the powder from a spoon or with your fingers without filtering it, you will achieve the effect of irregular, coarse, grainy sand (because powdered plaster contains larger grains which resemble small stones in 1/72); if you filter it through a strainer, you will obtain a more regular and smooth surface. My advice: experiment, experiment, experiment, after all powdered plaster costs around 1 euro per kg
Do not throw away hardened plaster: break it instead into little pieces, and those will make perfect desert stones, like the ones in the last image below.
Finally, paint it irregularly with a mixture of dirty white, sand, earth and dust colors and remember that a realistic look depends heavily on it. You can also add lichens or plants for a more realistic final look.
That's all, thanks for looking and let me know if you need some more insight
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