How to Make a Mold for Fiberglass

Written by melissa j. bell
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Fibreglass, although a fairly toxic substance, is a common choice among professionals, students and hobbyists for creating props and other art projects. In order to form fibreglass into the desired shape, it must be pressed into a mould, or negative impression of a sculpture. The best moulds for casting in fibreglass are made from plaster. They are usually made in the two-part style, since fibreglass dries too hard to peel out of an enclosed mould.

Skill level:

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Things you need

  • Water-based clay
  • Sculpting tools
  • Clear sealer spray
  • Modelling clay
  • Gypsum
  • Water
  • Bucket
  • Paintbrushes
  • Burlap
  • Scissors
  • Mold release agent

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  1. 1

    Sculpt your original design or model object from water-based clay. Coat the entire sculpture with a clear sealer, such as Krylon Crystal Clear, and let it dry completely.

  2. 2

    Visualise dividing the sculpture in half, creating two identical sides. Along this dividing line, build a thin wall of modelling clay that is a few inches tall.

  3. 3

    Mix together a small batch of plaster. Add gypsum and water to your bucket until you have a soupy texture. Stir the mixture to remove air bubbles.

  4. 4

    Paint an impression coat on one side of the sculpture, all the way up to the modelling clay. Let the impression coat dry to the touch.

  5. 5

    Cut your burlap into small strips. Paint another layer of plaster over the impression coat, then press pieces of burlap into the plaster.

  6. 6

    Paint more plaster over the pieces of burlap, then let this layer of the plaster mould dry to the touch.

  7. 7

    Add more layers of plaster and burlap until you have the mould thickness you like. Let this half of the mould dry completely.

  8. 8

    Repeat Steps 4 through 7 on the opposite side of the sculpture. Let the entire mould cure overnight.

  9. 9

    Peel the halves of the two-part mould apart, and scoop out the clay inside. Clean the mould completely with warm water, leaving no traces of clay behind.

  10. 10

    Coat the inside of the mould halves with mould release, so that the fibreglass does not stick to the inside. Let the mould release dry before you begin fibreglass casting.

Tips and warnings

  • Once finished with the mould making, you can apply resin and fibreglass directly into each mould half. To learn how to cast in fibreglass, see the Resources section.
  • You can also create a positive mould for fibreglass that works like a skeleton, using materials like wire and papier-mache. Simply drape the fibreglass over the positive mould instead of pressing it inside a negative mould.

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