How to Make a Fiberglass Mold: Step-by-Step Guide (Part 1)

Fiberglass molding is a great technique for duplicating parts, such as a boat hull. This guide walks you through the essential steps to create a fiberglass mold. In part two of this series, we will cover fiberglass layup to create your duplicate part.

Step 1: Mounting the Original Part (Plug)

1. Choose a Base – Use a solid, non-porous surface like coated hardboard.

2. Attach the Plug – Secure the original part (plug) to the base using a glue gun for easy removal later.

3. Check for Gaps – Ensure there are no gaps between the plug and base, as gel coat cannot bridge gaps.

4. Fill Gaps – Use modeling clay to seal any gaps, then trim with a knife for clean edges.

5. Inspect for Imperfections – Any flaws in the plug will be duplicated in the mold and final product.

Step 2: Applying Mold Release

Mold release is essential for removing the cured mold from the plug.

1. Apply Mold Release Wax – Use Partall Paste #2. If the plug is porous, seal it first.

2. Buff the Wax – Polish it to a shine.

3. Apply PVA Mold Release – Use a Pre-Val sprayer for small projects.

4. Layering the PVA – Start with a light mist coat, let it dry, then apply two heavier coats, ensuring each layer dries before applying the next.

Step 3: Applying Gel Coat

Gel coat forms the outer layer of the mold.

1. Choose the Right Gel Coat – Tooling gel coat is best for high-volume molds, but for limited-use molds, standard gel coat with black pigment works well.

2. Mix the Catalyst – Follow the manufacturer’s instructions.

3. Use a Resin Spray Cup Gun – Do not use a paint sprayer.

4. Spray the Gel Coat – Apply in long strokes in two directions for even coverage.

5. Check Thickness – Use a mil gauge to ensure a thickness of 15-20 mils.

6. Let It Cure – Curing time varies from 30 minutes to a few hours, depending on temperature and catalyst ratio.

Step 4: Creating Smooth Corners

Fiberglass does not adhere well to sharp angles, leading to air pockets and cracking.

1. Prepare a Putty Mixture – Combine Cab-O-Sil and TAP Bond Coat Resin to create a peanut butter-like consistency.

2. Apply the Mixture – Spread onto 90-degree angles and shape into a smooth cove using a stir stick or your finger.

Step 5: Applying Fiberglass Layers

1. Mix Resin and Catalyst – Prepare only what you can use within 20 minutes.

2. Create a Border – Use fiberglass selvage tape to reinforce the edges.

3. Apply the First Layer – Wet the surface with resin and lay fiberglass mat (3/4 oz or heavier).

4. Saturate the Mat – Use a dabbing motion with a brush for thorough wetting.

5. Use Small Pieces – Cut the mat into smaller sections for easier application, ensuring overlap with no gaps.

6. Use a Roller – An economy roller helps press the mat down and remove air bubbles.

7. Build Layers – Apply 3-6 layers for small molds; larger molds may require 6-10 layers or more.

Step 6: Finalizing the Mold

1. Apply a Surface Curing Agent – Ensures a tack-free surface for the mold.

2. Let It Cure – Allow full curing before attempting to remove the mold.

Conclusion

Creating a fiberglass mold requires careful preparation, mold release application, gel coating, and layering fiberglass properly. Following these steps ensures a high-quality mold that can be used for multiple reproductions. Stay tuned for part two, where we’ll cover the fiberglass layup process!