Real rocks are heavy. Because of this, it is sometimes easier to make fake foam rocks for projects, such as science fair projects or reptile enclosures, especially if you will have to transport the rocks to multiple locations. Another benefit to making fake foam rocks is that you can customize the rocks to fit your needs, which is more difficult with real rocks.
Create the Basic Rock Shape
Glue polystyrene foam insulation sheets together until you reach the desired thickness for your rocks. Spray craft glue works well but, a cool glue gun is also an option.
Draw the basic outline of the rocks on one the top of the foam sheet using a marker that is lighter than the color you will paint the rocks. Remember that rock outlines in nature are irregular, so if you want realistic looking rocks, you should make each rock different.
Cut the rocks out using a spiral saw. Don’t worry about mistakes or patterns that aren’t followed exactly, as no one will know about the mistake except you.
Use a melon baller to make craters in the rocks, if desired. Make them different depths and use different size melon ballers to make the craters look realistic.
Make cracks and crevasses in the rock using a butter knife. Make these slightly wider than you actually want them. Very thin cracks and crevasses won’t be visible on the finished rock.
Slice an old pair of sheer nylon stockings into strips. Apply these strips to the rock using spray craft glue. Use your fingers and cotton swabs to press the stockings into the craters, cracks and crevasses.
Apply a thin layer of grout over the stockings. Follow the manufacturer’s directions when mixing the grout but, add extra water a little at a time until the grout is thinner than normal. Allow this layer to dry for at least 24 hours.
Decorate the Rocks
Add sand, if desired, to the rocks to give them more texture. To do this, mix some grout exactly as directed on the instructions. It should be fairly thick. Spread the grout onto the rock. Drop handfuls of sand onto the wet grout. Spread the sand out by hand. You can use any color sand you want. Allow to dry for at least 12 hours.
Use a dry, wide paintbrush to ensure all the excess sand is removed. Shake off the excess sand or blow it off using a cool blow dryer.
Paint the rocks, if desired. Use an acrylic based spray paint for the base coat. If you want to add realistic markings, use a fine tipped detail paintbrush and acrylic paint after the base coat dries.
Apply a water-based sealant to the fake rock. If the rock is going to be used in a reptile enclosure, make sure it is non-toxic when dried.
Wear gloves when making fake foam rocks to protect your skin.