Artificial rocks are used in many different settings in place of heavy over-sized boulders or natural rocks. They are used in museums, businesses and residential homes for vivariums, landscapes, natural settings and other displays. Sandstone rocks, as well as other rocks, can be made out of different materials, including plastic and styrofoam. You can make large or small sandstone rocks with styrofoam, glue and a few more materials.
Decide on the shape and size of your artificial sandstone rock.
Cut chunks of styrofoam and glue the pieces together to make one large or medium-sized solid mass. Allow the glue to dry completely.
Sculpt your styrofoam. Use a utility knife to carve the styrofoam into your desired size and shape. If you need to cut large pieces of styrofoam, use your sharp flat knife to sever the pieces. Shape your styrofoam to look more like natural rocks by using heat from a soldering iron or a heat gun in a ventilated room. Wear a face mask to protect yourself from the toxic fumes. Leave imperfections in the styrofoam similar to natural rocks.
Prepare the outer surface of the rock with grout. Add acrylic polymer to some grout to stop it from flaking off as it dries. Prepare the grout according to the instructions; but make the consistency more watery than recommended by the manufacturer. Add the acrylic polymer to the grout.
Apply a light layer of grout with acrylic polymer on the styrofoam with a paintbrush. Allow the styrofoam to dry and add another light layer. Carve out grooves and imperfections while the second coat dries. Your next coat should be darker in colour and thicker; so, you may want to add a little more grout. The acrylic polymer will stabilise and hold the layer together while you apply the next layer. Add about two more coats after the third coat, allowing it to dry between coats.
Allow the grout layers to dry overnight. Add a final layer of grout. The consistency should be very thin and dark. Wipe away most of the final layer with a cloth, leaving it in the cracks and crevices. Use your wire brush to create a natural look. Some of the lighter layers should show through the dark layer. Allow the rock to dry thoroughly.
Cure your rocks. Add moisture to the artificial rocks for two to four weeks. Concrete (grout) cures by being wet, so don't forget this step. Allow it to dry.
Seal your rock (optional). Apply epoxy to the rock with a paintbrush for a shiny finish. Let it dry completely.