How to make a fiberglass mold with styrofoam

Updated February 21, 2017

Fibreglass, a material most often used for casting prop objects, is also useful in making moulds of plastic or styrofoam template objects. You might want to use fibreglass moulds to make copies of a small design, such as a model part or art piece, which can be first crafted in styrofoam.

Carve the piece of styrofoam for which you would like to make your fibreglass mould. Shave and finish the outside of the styrofoam object so that it is as smooth as possible, then coat the styrofoam in a clear sealer and let it dry.

Find the middle line of the styrofoam object and mark it out, from one side of the base, over the top, to the other side of the base. Along this line, build up a thin wall of modelling clay that is at least as tall as you would like your fibreglass mould to be. Smooth out the modelling clay so that the division between the two sides of your styrofoam object is as thin as possible.

Apply wax mould release to both sides of the styrofoam object and the modelling clay in several layers, buffing as you go along until your layers are fairly smooth. Apply a full layer of liquid mould release over the wax mould release, and let this layer dry completely.

Gather your resin, tooling gel, silica filler, fast hardener and graphite powder together in a protected, well-ventilated work area. Mix together a small batch of resin, following the instructions on your particular product. Add to this resin mixture a small amount of colloidal silica filler, fast hardener and graphite powder so that your resin will dry more quickly and effectively. Pay attention to the products' packaging recommendations.

Paint a little resin mixture into the grooves and corners on one side of your styrofoam object, then let the resin dry completely. Over the hardened resin, paint a full layer of tooling gel coat, still working on only one side of the styrofoam object. Let the gel coat dry for 30 minutes. In the meantime, mix together another batch of resin, tooling gel, silica filler, fast hardener and graphite powder.

Apply a coat of resin mixture over the dried tooling gel coat. Cut several strips of fibreglass cloth and lay them over this coat, pressing the fibreglass into the wet resin. Let the resin dry, then paint another coat of resin over the fibreglass cloth. Continue adding layers of resin and fibreglass cloth to the side of the styrofoam object you are working on until you have the desired thickness. Let the resin cure overnight, then remove the modelling clay and perform Steps 4-6 on the opposite side of the styrofoam object.

Pull the fibreglass mould halves off of the styrofoam, separating the halves at the centre split. Trim the rough edges of the fibreglass mould and clean it of all clay.


Never touch resin with your bare hands or let it get on your skin. Follow all safety precautions detailed on your product packaging.

Things You'll Need

  • Styrofoam object
  • Clear sealer
  • Modelling clay
  • Liquid mould release agent
  • Wax mould release
  • Latex gloves
  • Resin
  • Fibreglass cloth
  • Fast hardener
  • Colloidal silica filler
  • Graphite powder
  • Tooling gel coat
  • Bucket
  • Paintbrush and stirring tool
  • Scissors or Dremel tool
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About the Author

A writer with a Bachelor of Science in English and secondary education, but also an interest in all things beautiful, Melissa J. Bell has handed out beauty and fashion advice since she could talk -- and for the last six years, write for online publications like Daily Glow and SheBudgets.