Rocks without deep undercuts are best for casting rock molds
Making a fake rock look realistic is a difficult task, as the organic shapes altered by the forces of nature often appear too sculptural, smooth or patterned when recreated by the human hand. However, fiberglass rocks cast from molds taken from actual rocks can fool even the careful observer into believing they're looking at the real thing. A complex process, rock molds must first be created in rubber in order to get the mold to release from the real rock without destroying it. This process can be used on most small and medium-sized rocks.
Instructions
Making a Rock Mold
1. Brush one coat of latex molding rubber onto the surface of the rock with one medium-sized paintbrush, being careful to completely cover the rock area you wish to duplicate. Eliminate any bubbles in the wet latex, as they will alter and weaken the mold. Let dry.
2. Repeat Step 1 three more times, making sure to let each layer of latex rubber thoroughly dry before applying the next coat.
3. Embed the strips of cheesecloth into the mold by laying it into the fifth layer of wet latex rubber as you paint it on. Make sure that every inch of the mold surface is covered by cheesecloth, as your mold will weaken and tear in uncovered areas. Let dry.
4. Repeat Step 1 three more times over the top of the now-dry cheesecloth layer. Make sure to let each layer of latex rubber thoroughly dry before applying the next coat. At this point you should have eight layers of latex, including one with cheesecloth embedded into it.
5. Repeat steps 3 and 4, making the ninth layer a second cheesecloth layer, covered by several more layers of rubber. Let each layer dry before applying the next. When finished, you will have a real rock coated with 12 layers of latex rubber molding compound. Your rock mold is now complete.
Making a Mold Cradle
6. With the second, medium-sized paintbrush, coat the exterior of the rubber mold with a layer of fiberglass resin and embed the fiberglass cloth into it. This step will create a cradle for the rubber mold help the mold hold its shape. Let it cure.
7. Coat the fiberglass cloth with a second coat of resin and let it cure.
8. Repeat Step 2 two more times, letting each layer cure before applying the next coat.
9. Remove the hardened, fiberglass cradle from the rubber mold and turn it on its back, the opening facing upward.
10. Remove the real rock from the rubber mold by peeling it carefully away, taking care not to tear the rubber. Use gentle pulls to release any rubber that has settled into the rock surface and let pressure ease the mold out. Do not yank or you might destroy the interior surface of your mold.
Casting a Fiberglass Rock
11. Lay the released mold inside the fiberglass cradle, with the now-empty interior of the rock mold facing upward. Brace the rounded, fiberglass cradle with wooden wedges to keep it from shifting as you cast your rock.
12. Mix rock dust, dry cement or some other coloring agent into a portion of your fiberglass resin that will become the first coat. This will serve to give your rock an authentic gray, brown or red rock color as well as prevent the fiberglass cloth from showing through the resin.
13. Brush one coat of colored fiberglass resin into the interior of the rubber rock mold in the empty space that used to hold the original, real rock. Use the medium resin brush to paint the tinted, fiberglass resin. Let it cure. This layer is known as the gel-coat, and it must harden, or cure, before proceeding any further.
14. Brush a second layer of fiberglass resin into the rock mold and embed fiberglass cloth into the wet resin before it cures. Let it cure.
15. Paint two to three more layers of resin into the rock mold, letting each layer cure before applying another layer. Let the entire fiberglass rock cure.
16. Remove the rubber rock mold from its fiberglass cradle and gently peel the latex away from the fiberglass rock.
