The "Highwayman Inn" (from Sketchup and 3DS to Reality)

Step 10: Chapter 3- Terrain Modeling- 3: Backfilling the Terrain

 
  Now that you’ve roughed out the general shape of your terrain you will want to begin the process of filling the gaps between the rocks.  This will refine the topology and smooth out the elevation to make it look more natural.  Begin by filling the gaps between the rocks with joint compound.  Scoop out large dollops and apply them with a pallet knife.  It doesn’t need to be pretty or particularly nuanced.  You just want to fill in the space around your rocks.  Make sure however that you leave the top portions of your rocks exposed since it will give your terrain some character when you paint it later.  Daub the surface of your joint compound with a sponge after applying it.  This will break up some of the liquid smoothness of the compound and give the surface some “tooth” to better accommodate your next steps.  Allow the compound to fully cure for about a day in a dry place that is at (or above) room temperature.

  Now you will begin layering on Extra Coarse Pumice Gel to create the more pebbled aspects of the terrain.  Use a pallet knife to extract it from the can and use a wet brush to apply it to the stone and to the surface of the joint compound.  Apply this across your terrain with a focus on the gaps and allow it to dry.

  The terrain should now have a more pebbled texture after the Extra Course Pumice Gel has dried.  Coarse Pumice Gel will be used to add the last step of refinement; the soil layer.  Scoop some Gel onto a plastic pallet and lightly dilute it with some water.  Use a brush to apply the Gel with a focus on the joint compound and the remaining flat areas.  The logic is that you want to soften the feel of the filler and pebbles and diffuse the flatness of the board while preserving what remains of the visible rock. 
Construction of the terrain will be complete at this point.  The terrain will be very heavy and I recommend that you mold and cast a version that is much lighter.  You can make it stronger and lighter with the added benefit of being able to cast copies.