Once the structure is made, you have to cover it. For an earthy or muddy ground, the cheaper way is to use sawdust. Of course, you have ta make it but if you use a jigsaw to make the base, you do. First, you must mix the sawdust with diluted white glue. Then, you can apply this mix directly on the primer coat. After drying, you get a crust easy to paint with an airbrush. The last stage is usually a drybrush work on the irregular surface.