Stone and rock add weight to your designs, whether it be an interior wall, or a rock wall for a water feature in your landscape. Creating convincing faux rock requires study of the real thing. Find a sample of the stone you wish to imitate and a picture of that stone as used in a wall. One of the easiest and cheapest ways to create faux stones is with carved styrofoam. This will be a messy project, so wear something you are prepared to sacrifice.
Cut your stones from the styrofoam. Use your sample as a pattern and try to imitate rather than duplicate, not all of your stones should look alike. Cut the rocks from a sheet of 1½-inch styrofoam insulation. This gives enough depth for detail while giving you a flat surface to glue the rocks to the wall underneath. Use a jigsaw to cut the basic shape out. Use a utility knife, grater plane or almost any other cutting tool to shape the stones. Use your sample as the model and try to imitate the surface.
Paint your rocks with a base coat of a medium tone from your sample. Exterior latex paint works best. Choose your gloss level according to the rock sample. Gloss level range from flat to high gloss. You will also need a highlight, or lighter color and and shadow, or darker color, of paint as well. Choose these from the stone sample. Apply the paint with a disposable brush. Allow the paint to dry.
Adhere the rocks to your wall with latex construction adhesive designed for foam. Start at the bottom and space the rocks according to the wall picture you selected. Cover the entire wall.
Create a mixture from the remaining base color and white school glue, one part glue to two parts paint. This will act as your topcoat and seal your styrofoam. Cut cheesecloth into a square 8 to 10 inches across. Paint a layer of the glue mix over the stones and paste the cheesecloth to the stones with it. Press the cheesecloth into the mix and cover it with another coat of glue mix, using a disposable paintbrush.
Add highlight and shadow with a sponge while the glue mix is still wet. Blend the three colors across the stone; using your sample as a guide, try to mimic the texture of your stone. You will find that as they blend you will end up with more than three shades and a realistic color.
Apply highlights and shadows to the rocks with a brush. Use the highlight colors to brush onto the tops and other areas that catch the light, while the shadows go to the bottom and any hollows or divots you cut in the stone's face. Allow the paint to dry.
Add at least two coats of clear water-based polyurethane finish. Choose a gloss according to the look of the stone. They range from flat to semigloss. Apply the polyurethane with a clean disposable brush.