How to Make Fake Rocks Using Expandable Spray Foam

Cover the work area in a well-ventilated space -- ideally outdoors -- with a sheet of cardboard or a plastic tarp. If using cardboard, make sure the cardboard is larger than the rock you intend to create.

Shake the expandable foam can for 30 seconds or as recommended on the label, as this may vary by brand or product.

Put on a pair of rubber gloves and attach the nozzle or thin straw to the spray valve on the can. In some cases, this may already be attached to the can.

Spray the foam slowly into a small spiral atop the cardboard or top, much like adding a can-based whipped topping to a dessert. Continue spraying slowly to create a rock-shaped mound, noting the expansion happening during this process. Stop spraying well before the foam reaches the desired rock size to wait for the expansion to occur. Once expansion stops, spray more onto the structure, as needed.

Shape the foam gently with your gloved hands or a putty knife while it is still wet or tacky -- usually during the first five minutes after spraying. Allow the foam to cure for eight hours or as recommended once the basic shape is in place, or if shaping proves too messy while the foam is wet.

Shape the foam once it has cured using a rasp to remove small hunks of the material or a bread knife to slice larger chunks away. Use the rasp to smooth small imperfections.

Paint your finished faux rock, if desired, with several shades of the same basic rock color for realistic effect. Pour a bit of tan and brown or dark gray and white acrylic paint, for instance, onto a disposable plate. Dip a sponge or rag into parts of each color, then dab it all over the faux rock. Use another rag or paintbrushes to blend the colors slightly on the finished rock for a natural look.