Things You'll Need
Acrylic paint in rock colors
Containers for paint, such as empty sour cream tubs
Sponges (cellulose dishwashing sponges with many holes in them work well)
White acrylic paint for first coat, if needed
Old butter knife
Artificial rock creation involves shaping materials such as paper mache, foam or Styrofoam to mimic natural rock. The elements that make an artificial rock a convincing replica of an actual rock include irregular shape, natural colors and believable texture. An effective paint job on an artificial rock gives the appearance of surface irregularities and dimension. With basic tools for applying paint to artificial rocks and a selection of paints, you can create a realistic paint job to bring your artificial rock to life.
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Select paint colors to create the type of rock you desire. Choosing more than one shade of each color—or blending the colors yourself—helps to create a textured appearance. For example, to duplicate the effect of stripes of sediment in red sandstone, use a dark rust and a light rust paint.
Cover the work surface with newspaper. Set out the paints, containers and sponges.
Paint the artificial rock with white first. Dip the paintbrush in the white paint. Stroke the paint across the artificial rock using even strokes. Continue to cover the artificial rock. Allow the primer coat to dry.
Pour a half-dollar-size pool of each color of paint into your containers. Mix needed colors, such as white and black to make gray. Mix the paint with an old butter knife.
Dip a sponge into the main color you chose for your artificial rock, such as dark gray. Wipe the sponge at the edge of the container to remove excess paint. Press the sponge onto the artificial rock surface. Because of the holes in the sponge, the paint will go on in irregular patterns.
Repeat this process to add the first color all over the top and sides of the rock. To keep from smearing the paint, leave the underside of the rock unpainted for now. Allow the paint to dry.
Dip a fresh piece of sponge in the second color. For example, the artificial rock will be dark gray with a pattern of lighter gray and white. Paint the artificial rock with the light gray paint by pressing the sponge lightly on all the exposed areas. Allow the second paint layer to dry.
Dip a third clean piece of sponge into the white paint and dab it on. Use the third color sparingly, so that it highlights the rock and doesn't overwhelm the main color. For example, apply light dabs on the most prominent parts of the rock. Allow to dry.
Flip the artificial rock over and repeat Steps 5 through 8.
To work on small artificial rocks, cut the sponges in half. Rocks a foot in diameter or more will be faster to paint with the full-size dish sponge.
If the artificial rock is large or rounded, place it in a bowl to help stabilize it while you paint.