To make larger rock formations, obtain some nonrusting window screen. Over this you will spread a mixture of Durham's Water Putty and water.
Mix this up, adding the powdered putty until you get a substance about the consisting of cake icing. You spread it over the screen, which you form to the shape that you want, tacking it down to the base with thumbtacks. Make wrinkles in the screen to get variation.
Once you spread the putty onto the screen, it will take about an hour to set up to where you can work with it. When it has a leathery texture, you can use your sculpting tools to form it. You can also mix up some fairly stiff putty and add it to areas that you want raised and the like. Work with this a bit. When dry the putty will be rock hard and very durable.
To get some natural looking texture, you can use bits of coal spray coated with enamel paint (black is best) which you can press into the putty while it is still wet, preferrably before it becomes leathery. When it dries like you like it, paint the entire thing black or dark brown and drybrush on the color that you want, progressing lighter and lighter with coats of grey or brown until you get to white, each color being used more sparingly than the former until you get to the white, which will be drybrushed VERY lightly to the very tops of the surface detail. This will give a natural looking rock effect.
When dry, the putty will add a bit of weight to the piece, so if that is a problem, it is better to work smaller and use more lightweight coal sprayed with enamel. I even did a piece with Durham's Water Putty that was a hollow rock formation with a cave inside, which was to be filled with treasure. A mother dragon sat atop the rock with a baby dragon at her side.
You can do a lot with dioramas to make your figures look like something out of a fantasy novel. It just takes time and patience. Model railroad magazines can be a good source of information on how to do landscaping and realistic trees and undergrowth. If you have no intention of using the piece in game play, you can get as elaborate as you want with the size and weight of a piece.
If you want the look of dirt, spread a mixture of water putty over the base and then sprinkle the powder straight onto the mixture and let it dry. When the powder is sprinkled onto the putty, make sure that the surface of the putty is wet, causing the powder to stick and saturate. When dry, thump the base to shake off any excess powder.
The dirt texture can be painted, washed, and drybrushed to get a good, natural effect. Flock, available at model train or hobby shops can be added by applying glue to the dirt textured surface and then sprinkling the flock onto the wet glue. Again, experimentation is the word here as well.
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