How to Model & Make a Fiberglass Mold

by Leah Newman
This bumper was most likely cast from a fiberglass mold.

This bumper was most likely cast from a fiberglass mold.

Fiberglass can be applied to almost any surface, and the surface can be reproduced using the dried and hardened fiberglass wrapping as a mold. Fiberglass molds can be cast to make reproductions in clay, plaster or almost any other modeling material. While fiberglass molds are not typically used for intricate or detailed work on a small scale, they are commonly used for making large models, as well as items like boats and car bumpers. The molds are strong and can be reused.

1. Prepare the item on which fiberglass will be applied. Spray a thin coat of mold release wax on the surface of the object. Allow to dry for a half hour. Apply a second coat, ensuring all areas are covered. This coat may be thicker than the first. Allow to dry for at least 30 minutes. Apply a third coat, and allow to dry.

2. Use a soft brush and apply a thin layer of polyester gelcoat to the surface. Completely cover the surface, checking for gaps or brush marks. Allow to dry for 2 to 4 hours, then repeat. Allow this coat to fully dry, then add a final coat. Allow this coat to dry for a little less than two hours, leaving it tacky.

3. Cut strips of the matting, and press them on to the tacky gelcoat. Overlap the strips well, ensuring there are no gaps.

4. Dip a brush in the polyester resin, and work the resin into the matting by dabbing the brush on the surface. Continue until resin completely covers all matting, and is worked into the surface. There should be no visible white fibers and no air pockets. Drying of the resin is dependent on how thick the surface is, but typically takes about 3 hours.

5. Add two to three additional layers of matting and resin, allowing it to cure between layers.

6. Allow the fiberglass to cure for two to three days, then gently remove the original from the mold. If you cannot remove the subject using your hands, a wooden stick or plastic wedge can be used. Avoid metal tools when working with your fiberglass mold.

7. Sand the interior of your mold with a medium-grade sand paper. Spray a thin layer of mold releasing wax into the mold, and allow to dry. Repeat with at least two more coats and allow to dry completely before casting the first time. Add at least one layer of the mold releasing wax each time you use the mold.

Items you will need

  • Polyester resin
  • Chopped strand matting
  • Polyester gelcoat
  • Disposable paint brushes
  • Aerosol mold release wax spray


  • The chemicals used to make fiberglass can give off harmful fumes. Always wear a face mask with vapor filters when making fiberglass.

About the Author

Leah Newman has been a professional writer since 1999, writing about fine arts both in print and online. She specializes in how-to articles covering DIY projects. Newman holds a Bachelor of Arts in English from the University of Georgia and a Graduate Certificate in Children's Literature from Pennsylvania State University.

Photo Credits

  • Jupiterimages/ Images