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Clinic - Rock Mold Casting
1. Sources of Latex Rubber Molds Commercially available from Woodland Scenics, Home Made or borrowed from a friend.
 
2. Making Your Own Mold Items Needed Liquid Latex Rubber such as Mountains in Minutes or Woodland Scenics or other brands. Available at Hobby Shops and Craft Stores. A rock with good detail or even better, a large lump of coal. Coal normally has very good detail in the right size for our modeling purposes. Gauze or old pantyhose to reinforce the mold. A cheap, disposable nylon paintbrush inch or smaller. Most people will tell you the brush will be ruined but if you use nylon and wash it with soap and warm water it can be reused.
 
3. Let's make the Mold Wet the rock or lump of coal with a spray bottle of water with a drop or two of liquid dish soap. The water helps the rubber to get into the small cracks and detail. Dip the paint brush into the liquid latex and "paint" a coat of rubber onto the face of the rock/coal. When you have finished painting the face of the rock/coal (a mold about the size of your hand is much more manageable than a larger one.) Wash your brush and allow the rubber to dry according to the product instructions (usually about 24 hours.)

You can speed the drying time by using a hair dryer on the "warm" setting - do not use "hot". As the first coat is drying do not be concerned if it turns almost clear in color.  When the first coat is dry, "paint" on another coat of rubber directly on top of the first coat. You will not need to put water on the mold for this or any following steps. Wash your brush and allow the rubber to dry.

When the second coat is dry, apply a third coat as you did the second coat. Wash your brush and allow the rubber to dry.

When the third coat is dry, apply a fourth coat and immediately press some gauze or pieces of old pantyhose into the wet rubber and immediately apply a heavy coat of rubber onto the gauze/pantyhose. Wash your brush and allow the rubber to dry.

When the rubber is dry gently peel it off the rock or coal and you have a mold, ready to make rock detail.
 

4. Adding Rock Details to the Scenery When you have the hard shell of your landscape in place, determine where you want to install a rock casting. Apply some of the detergent/water from your spray bottle to the area where the rock will be. This will keep the old surface from pulling the water from your new casting and making it weak. Spray a little of the water into your mold to wet the inside surface and mix up some Hydrocal or Molding Plaster (Some prefer plaster while I prefer the Hydrocal.) Mix the material to about the consistency of pancake batter or maybe a little thinner. Lay the mold on a flat surface and pour the mixture into the mold. After a few minutes you will see small cracks develop on the surface if you pick up the mold and flex it backward slightly. When it reaches this stage, it is ready to apply. Press the mixture in the mold against the wet surface of your scenery. Press the edges down tightly and hold the mold in place. After a few minutes the mold will become warm as the material begins to harden. Gently peel back the edges of the mold to make sure the material does not chip or break away from the hard shell. If it does, press the mold back and continue to hold it on place a while longer. If not, continue to peel the mold off revealing your new rock casting. It is best to use several molds so all your rocks do not look the same. It is possible to only use a portion of a mold to change the appearance of the rock casting and the mold may be rotated to give a different look. Color the rock by wetting the surface then painting on some acrylic paint but not covering the entire face of the rock. Spray some water on the acrylic paint and it will flow into the cracks and crevices. Apply ground foam and other scenic products to finish the area.

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