How to Make Spray Foam Rock

Choose a flat location for constructing the spray foam rock. Place wax paper on the work surface. Use wax paper, as it will release from the spray foam and protect the work surface.

Create a Styrofoam core for the spray foam rock. Choose blocks of Styrofoam and glue them together with construction adhesive. Allow the adhesive to set for at least one hour before proceeding.

Place the glued Styrofoam block on the wax paper. Carve a rough shape for the rock with a sharp knife, or consider using an electric carving knife for larger projects. Leave rough and uneven edges to add a realistic effect and provide more surface for adhesion to the spray foam.

Apply spray foam to the entire surface of the Styrofoam. Work quickly, as the foam will begin to expand and cure almost immediately. Smooth the foam with a trowel into the rock shape you want.

Allow the spray foam to harden and cure for at least one hour. Add realistic colors and effects to the spray foam rock with spray paint. Use black, brown, green and gray, for example, to add a realistic finish to the spray foam rock.