Wax your entire fiberglass form with several coats of hard wax or mold release, making sure every section is thoroughly covered. Don't miss any spots, or your final fiberglass prop will not release from the mold.
Choose the location where the mold will divide in half and build a clay dam at this juncture. The dam should be at least 1 inch tall and run the length of your form and down both sides. This is where your mold will separate to release your finished prop. Take into account any crevices that might catch when releasing your mold when you select the location of your dam.
Coat 1/2 of the object with fiberglass resin (up to the dam). Embed fiberglass cloth in this resin. You will need at least four to five coats of resin & cloth.
Remove the clay dam and repeat step 3 to build the other half of the mold.
Remove the original fiberglass form from inside your newly created fiberglass mold and bolt the two halves of the mold together.
Your fiberglass mold is now ready to use. Wax the inside of the mold and repeat the process detailed in step three, coating the entire interior with alternating layers of resin and fiberglass cloth.