I've now reached the stage where, once the glue is dry, I can give all the boards their black basecoat. I have applied the second (partial/patchy) layer of sand to create a bit of depth, while still retaining a mostly flat surface. I have also added some patches of rougher, but still faitly low-profile, vegetation using loose leaf tea (as I do on my figure bases) and some small scenic rocks. These will add a bit a variety when painted and break up the otherwise featureless terrain.
Painting everything with a wash of black is going to be tedious and messy, but it'll be progress!
Second layer of sand
Patches of vegetation and rocks. I could stop now and call it North Africa! :)