When it comes to fake substitutes for real stone, Styrofoam is a classic material choice used widely in stage and cinema productions. Though significantly softer and lighter than stone, and essentially not durable at all, Styrofoam closely resembles stone's textures when properly painted. Use the right colors and the right layering and painting techniques to recreate the look of aged stone, whether you're making a few simple rocks or a fake stone statue.
Make some cuts and breaks in the Styrofoam to distress it. Snap off pieces with your hand. Use a craft knife to cut small cracks and gouges; these will have the proper texture of broken stone once painted.
Give the Styrofoam a coat of acrylic spray primer; this will ensure that the subsequent layers of paint stick to the material with an even coverage. Let the primed Styrofoam dry according to the manufacturer's instructions.
Spray the Styrofoam with a layer of light gray spray paint. Let it dry.
Mix a solution of 1 part dark gray acrylic paint and 1 part water.
Work some dark gray acrylic and water mixture into the cracks and crevices in the stonework using the rag.
Apply irregular splotches of dark gray paint (with no water) using the plastic sandwich bag in place of a brush. Crumple the bag slightly to get the proper texture.
Run the feather in randomized, semi-swirl patterns through the wet, dark gray paint to create marbling lines.
Spray the Styrofoam with a layer of varnish to give it sheen, resembling the appearance of polished marble.