Some of the buildings are rendered with much the same stuff as a real building - plaster.
The material
Normal Polyfilla is used, simply because it is readily available and is easy to use. It is spread onto model with an artist’s palette knife, not particularly small as it needs to carry up to half a teaspoon of filler.
The method
If required, use masking tape to protect areas, spread the filler thinly like butter.
This is an ideal model to learn the process, the Wills SS30 Barn is cheap and easy to find. One consideration is the paint the building before applying the filler, it is easier to remove the wet filler than the paint from the dry filler.
First photo
Once applied, let the filler harden for at least 24hrs
Second photo
Smooth the filler with rough abrasive paper until the necessary finish is achieved. Please note I had forgotten to paint the building…..
Third photo
Apply a wash of a suitable dilute water colour, I use yellow ochre to replicate lime or sandstone. Weather the entire building with a dusting of off-white poster colour powder, WW Scenics offer Desert Yellow.