Hardening styrofoam, or extruded polystyrene foam, is one Hollywood's greatest tricks. The rocks and bricks of most sets are carved from styrofoam and then artificially hardened. Because it is so easy to shape, styrofoam is a near-perfect craft medium, except for its lack of durability. Learn to make styrofoam hard and you can create decorations, stage sets, and styrofoam models like the Hollywood pros.
Prepare the surface of the styrofoam using sandpaper. The idea is not to smooth rough patches but to add a subtle texture to the entire surface to allow for adequate gluing.
Cut fibreglass cloth to cover the entire surface of the styrofoam. The cloth will be used in a hardening state, so experiment with how it will lay on the styrofoam model. Any bunches or loose spots in the cloth will be transmitted to the model, requiring additional shaping.
Coat the entire surface of the styrofoam in model glue. Use a small brush to lay an even coat. Although any kind will do, working with a glue specially designed for full coatings, such as Mod Podge (available at craft stores), will give you a little extra time to lay the fibreglass cloth.
Lay down fibreglass cloth over the entire surface of the styrofoam object. Fibreglass cloth serves as an excellent sealant against moisture and other forms of damage. Allow to dry overnight.
Seal the entire surface with fibreglass resin. Fibreglass resin will harden the fibreglass cloth, forming a hard and durable surface.
Reshape where necessary. Any areas where the fibreglass cloth or resin has affected the shape can be sanded or smoothed back into the appropriate shape.
Apply fibreglass resin in a well-ventilated area.