Coat your model, usually a finished sculpture or existing non-porous object, in a mold release agent. If your model is a soft clay sculpture, either bake the sculpture or coat it in Krylon Crystal clear, if you do not wish to save the model. Along the middle line of the model, build up a wall of modeling clay from one side of the base to the other. Make sure that the modeling clay wall is at least as tall as you would like your mold to be.
Mix together gypsum and water in a bucket to create a smooth plaster mixture, and apply an impression coat to one side of the model, all the way up to the modeling clay line. Let the impression coat cure, then apply a second layer of plaster. Over this wet second layer, lay strips of burlap to soak up the plaster.
Let the second layer dry, then continue applying layers of plaster and burlap until you have a thick, strong mold. Let this side of the model cure, then repeat the process on the opposite side.
Pull the hardened plaster carefully away from your model, separating it at the modeling clay line. Clean all of the clay out of the mold halves, then coat the inside of the mold with paint. Let it dry completely.
Apply a mold release agent to the inside of the mold. Polyvinyl alcohol is the best mold release agent for fiberglass molding. Let the mold release agent dry before moving on to the casting stage.
Put on a vapor mask and a pair of latex gloves, then mix up a small batch of laminating resin with its catalyst, following the product instructions on the resin container. Paint an impression coat of resin into each mold half and let it set.
Cut a few small strips of fiberglass cloth and press them into the mold halves, letting them cover the impression coat. Paint a second coat of laminating resin over the fiberglass cloth, letting the cloth soak up the resin. Let the resin set.
Apply another coat of laminating resin and fiberglass cloth strips, building up the cast until you have the desired thickness. Let the casting cure partly, for at least a few hours, until the fiberglass is firm, but still flexible. Pull the fiberglass from each mold half, breaking the seal between the mold and the fiberglass, then replace the pieces back into the mold halves. Let the fiberglass cure completely.
Cut the excess fiberglass around the casting pieces and line up the edges of each half, so that they match properly. Tape the edges together with a few pieces of masking tape, and cover the seam with a thin layer of resin and fiberglass cloth. Remove the masking tape as you work on the seam.
Let the fiberglass cure completely, then put your particle mask on and file or sand down the seam. When finished, clean your work area with acetone to remove any spilled resin.