Create the sculpture from modeling clay, and coat with petroleum jelly. Using 2-by-4-inch lumber or plywood, make a box 3 inches larger than your model on all sides.
Mix plaster, and fill the box to a depth of 1 inch. Center the clay model in the box, and continue pouring plaster around the model. Allow the plaster to dry, and remove the clay.
Coat the inside of the plaster mold with shellac, and allow the coat to dry overnight. Brush paste wax along the inside of the mold to prevent the fiberglass from sticking to the plaster.
Mix Bondo with a few drops of fiberglass resin, and add the catalyst according to package directions. Use a paintbrush to paint the Bondo on the inner surface of the mold. The Bondo takes about 30 minutes to dry.
Mix the fiberglass resin with MEKP at a ratio of seven drops of MEKP per tablespoon of resin. Paint the resin mixture on top of the Bondo. Cover the resin with squares of fiberglass matte, making sure the sections of matte overlap. Brush on additional resin until the fiberglass matte is soaked. Let the fiberglass layer dry for 45 minutes or until it is cool to the touch.
Use a hammer and chisel to break off the plaster mold. Paint the fiberglass sculpture as desired.