I would put the spack down. Let it dry. Think about the order of diarama development. Depending upon how you are putting things together, you might wish to put the sandpaper (road) down first with glue. Then the spackle to hide edges of the sandpaper. Then your major structures blended in with more spackle. Then your trees (if any). Then paint your spackle to hide the "white". Then put your grass down with Elmers white glue (dries clear not yellow and allows your colors to show through). Then your diarama models and equipment.
You always have to think about the process.