Making a Fiberglass Retaining Shell

Overview
This procedure summarizes how to create a fiberglass mother-mold over a two-part polyurethane mold. Gelcoat is used as an undercoat next to the polyurethane mold to protect the mold from fiberglass particles. Fiberglass/resin is applied next, creating the durable, hard mother-mold that is lighter than a plaster mold. All of these procedures should be performed in a well-ventilaetd space using an appropriate solvent fume mask (protecting against styrene). Care should be taken when handling fiberglass as the material is highly irritating to exposed skin, eyes, and the respiratory tract.

Required Materials

• Mold releasing agent: Have successfully used SynAir 531 on polyurethane rubber molds.
• Fiberglass mat: Prefer the loosest weave possible and there are differences between brands. Napa AutoParts mat version works, but is not as good as others. Fiberglass cloth may have special uses, but is generally not preferred. Have successfully used the following brands: Napa AutoParts Mat.
• Gel coat: Have successfully used the following brands: TBD
• Laminating Resin: Have suceessfully used the following brands: Napa AutoParts.
• Acetone: Used to clean brushes and cans.
• Clean metal cans: 1qt size works well.
• Cheap 1 inch paint brushes: Have extras around in case they are needed.
• Gloves: You'll use more than you realize, so stock up before starting.
• Fume mask: Spend the money and protect your brain.

Procedure

1. Spray the polyurethane mold with mold release agent (531) prior to application of the gelcoat. Fill in any gaps or undercuts with clay.
2. Create 1-2” aluminum wedges (L-brackets). Align two brackets so the vertical faces are joined and drill a hole to allow a ¼” machine screw to pass through. These are a matched pair and should be kept together by joining them with a screw and nut. You’ll need enough for 2-4 pairs for each part of the mother mold (say 6-8 pairs for a two-part mold). Alternatively, you could create clay dams along the edge of each piece.
3. Add the hardner to the gelcoat and mix per instructions. Coat the surface of the mother-mold. Embed the aligned wedges along the border of each part of the mother-mold as the gelcoat hardens. Use a heat lamp if you want to speed the hardening time. Scrape away gelcoat from the border line between two parts of the mother mold to make it easy to separate the two edges later on. If you don’t apply the wedges now, you can add them later with bondo (polyester glazing putty) or 5-minute two-part epoxy. If working with clay borders, apply the gelcoat to the surface of the clay damto create a perpendicular wall along the mold surface.
4. When the gelcoat is relatively hard, can apply the laminating resin to the surface as a relatively thin coat. Apply fiberglass mesh that has been loosened by hand to allow greater surface area contact with the resin. Don’t brush the fiberglass, but use a patting motion to place the fiberglass where you desire it. Apply mesh with cut edges along the border of the two pieces to simplify piece separation later. If using wedges, apply the fiberglass material over the wedges and fill in any gaps under the wedges. If using clay dams, apply the fiberglass up the perpendicular surface of the dam on top of the gelcoat.
5. Apply the fiberglass in layers to gain the desired thickness (1/4” to 3/8” should typically suffice for a medium-sized piece (say less than 1.5 cu. ft). Can apply a second layer of gelcoat to protect the outer surface of the mold to protect against future handling.

Contributors: Rafael P, Mark W.