Set up a safe workspace outdoors or in a well-ventilated room. Tape trash bags or other plastic over any tables and the surrounding floor. Keep acetone in the room to remove any resin stains.
Put on your protective gear for working with fiberglass. Wear latex gloves, safety goggles and a vapor respirator, so that you do not come into contact with the toxic resin or its fumes. Make sure that the skin of your arms and legs are covered.
Coat your rapier prop piece with two layers of Bondo body filler, if you are using a Styrofoam prop. This will protect the material from dissolving. Let the Bondo dry, then lay the rapier flat on a work surface.
Pour a little fiberglass resin into a mixing container. Add the hardener as directed on your product's packaging. Stir the mixture well.
Cut a piece of fiberglass cloth that is slightly larger on all sides than your rapier prop. Lay the piece of fiberglass on top of the rapier.
Drizzle the resin onto the fiberglass cloth until it becomes "wet," or sticky. Spread the resin around with a squeegee, pressing the squeegee hard against the prop rapier so that it helps the fiberglass cloth stick to the prop.
Push any excess resin off the sides of the prop with the squeegee, onto your covered work surface. Let the prop dry for 24 hours, or as needed.
Turn the rapier prop over to the uncovered side. Repeat the fiberglass coating process, making a new batch of resin and "wetting" another piece of fiberglass cloth against the prop. Let the prop dry for 24 hours again.
Clean up your workspace by throwing out all damaged plastic and wiping up any resin spills with the acetone.
Swap out your vapor respirator for a particle respirator, and wear your safety goggles and gloves again. Move back into your safe workspace.
Smooth out the surface of the fiberglass rapier using a power sander, and take off any excess fiberglass material.
Sand the rapier prop all over with sandpaper until you have a completely smooth surface.