Well after having come through a number of problems when it comes of diorama bases -I don't mean the wooden ones around the scene so that the borders look good, more temporarary borders you have to set so that either resin or plaster don't fly away - I set up building a 1mm thick plastic box.
Inside this box I will pour the plaster, the resin etc.. really the problem is that the plaster while setting sort of deforms the borders. So does the resin of course, so one of the tricks I use is to first lay a thin plaster coat at the bottom of the box so that I can ensure a complete waterproof box. And then I will pour resin layer above resin layer so that there would not be too much heat -well not much a problem with epoxy resin- or pressure against the borders.
So first the plaster -the only trick is *don't stir well*
With the plaster still half set, I proceeded to inprint some tyres marks as well as a shape looking like an explosion on the top left of the scene. there will be water and mud inside.
then Lifecolor and Prince August paints for the mud area (yes purple ! )