If you need rocks to use in scenery for a play or you want to add some interest to your flower garden, make rocks from styrofoam. Because they are lighter than real rocks, styrofoam rocks are much easier to handle. The styrofoam base is covered with a layer of sanded grout that is normally used to fill the spaces between floor tiles. Grout comes in various colours, including many rocklike shades of grey, tan, brown and terracotta. Use sheets of styrofoam to make smooth, flat river rocks or bumpy ledge stone. Styrofoam blocks work well for making bricks or small boulders.
Cut and carve styrofoam into the basic desired shapes using a utility knife or electric knife.
Mix powdered grout with water and stir thoroughly to achieve a thick paste consistency. Let the mixture sit for 10 minutes, during which time it will become somewhat thicker. Mix again.
While wearing rubber gloves, use your hands to spread a layer of grout over the styrofoam. Put it on thin and smooth or thick and bumpy, depending on the desired finished look. Allow to dry.
If the rocks will be placed outside or will need to be cleaned, apply sealer to make them water-resistant. Wait two to three days until the grout is completely set and cured, then brush on sealer using a foam paintbrush.
These rocks are for decorative purposes only. They are not strong enough to support a person's weight and should not be used for a rock-climbing wall.