The Proper Way to Lay Fiberglass on a Mold

Fiberglass parts are created in molds.

Fiberglass parts are used in products and machines everywhere in our daily lives. These parts are built up layer by layer in molds, a process which may take more than a day for larger projects. For quality work, you shouldn't simply slap the fiberglass and resin in place and walk away. It is important to lay the fiberglass properly to avoid air bubbles, weak spots, cracks and other imperfections.

Things You'll Need

• Mold
• Mold release
• Gelcoat
• Tint
• Mixing bucket
• Mixing stick
• Brush or spray gun
• Fiberglass mat or strand
• Scissors or chopper gun
• Fiberglass resin and catalyst
• Brushes or rollers
• Fiberglass cloth
• Air saw
• Sandpaper

Instructions

1. Prepare the mold with a release agent. Use a spray-on silicone release for molds make of silicone or rubber, and brush on a thin coat of paste wax for molds made of rigid materials such as cement and fiberglass. Buff any residue out after a few minutes.

2. Pour gelcoat into a mixing bucket and add tint if the product you are making needs to be colored. Mix the tint in thoroughly to avoid any color striations.

3. Add catalyst to the gelcoat following the mixing ratio provided on the gelcoat container. Mix the gelcoat quickly but thoroughly.

4. Add the catalyzed gelcoat to a gelcoat spray gun.

5. Spray the gelcoat onto the mold, building up a thickness of 1/5-inch.

6. Allow the gelcoat to cure for several hours. When you can not easily scratch it with your fingernail, it is ready for fiberglass. While it is curing, proceed with the next step.

7. Cut fiberglass mat into 5-inch squares. If your mold is particularly large, feed a roll of fiberglass strand into a choper gun.

8. Mix a batch of fiberglass resin with catalyst.

9. Brush a layer of catalyzed resin onto the hardened gelcoat.

10. Cover the mold with fiberglass squares, overlapping them slightly to ensure that there are no gaps. For larger molds, use the choper gun to spray a layer of chopped fiberglass fibers over the mold surface.

11. Saturate the fiberglass with more resin, either with a brush for smaller molds or with a roller for larger ones. Work out all trapped air bubbles, which will weaken the finished product if allowed to remain.

12. Continue building up layers of fiberglass and resin until the mold has reached the desired thickness. Smaller molds can be as few as four layers, while large ones may require eight or more.

13. Reinforce larger molds with a layer of large, overlapping fiberglass cloth squares. The specific size depends on the size of the mold, but they should never be too large to be unmanageable. Saturate this layer in catalyzed resin as you did the others.

14. Allow the fiberglass to cure. This may take a few hours or may require a full day depending on the size of the mold.

15. Remove the cured fiberglass from the mold.

16. Trim off any rough edges with an air saw.

17. Sand the outer surface with 100-grit sandpaper to remove any burrs or sharp protrusions.

Tips & Warnings

• Gelcoat can be brushed on if you do not have access to a spray gun. Brush on a thin layer, then let it cure for 4 hours before adding a second layer. Allow that layer to cure as well, then add fiberglass.

• Wear protective gloves when working with fiberglass resin.

• Wear goggles and a particle mask when cutting and sanding fiberglass.

• Work in a well ventilated area.

References

• Club Kit Clear Cote: Hand Lay-Up of Fiberglass Parts in a Mold
• "The Prop Builder's Molding & Casting Handbook"; Thurston James; 1989