How to Form Fiberglass With Plywood Molds

Make simple fiberglass shapes with a plywood mold.

Plywood can be used as a mold for fiberglass when you need a relatively simple shape, such as a box. The mold is assembled out of plywood, then fiberglass is cast within it. Once the fiberglass cures, the mold is removed from the finished cast. Since both the mold and the finished product are rigid forms, special care must be taken to prevent the two from locking together.

Things You'll Need

• Plywood
• Wood screws
• Screwdriver
• Acrylic enamel spray
• Paste wax
• Paintbrushes
• Paper towels
• Fiberglass mat and cloth
• Scissors
• Disposable buckets
• Fiberglass gel coat and catalyst
• Fiberglass resin and catalyst

Instructions

1. Construct a mold out of plywood and wood screws. When building a box, your mold will begin with a bottom and four sides. If you need all six sides of the box, the final side must be done later as a sheet of fiberglass that is cut to fit the box and attached with additional fiberglass.

2. Seal the inside of the mold with several coats of acrylic enamel spray. This will prevent the fiberglass resin from seeping into the porous wood. Keep the coats thin to prevent dripping and allow each coat to dry before applying the next.

3. Brush the inside of the mold with a coat of paste wax to prevent the fiberglass from sticking. You only need a micro-thin layer, so wipe away any excess with a paper towel. Make sure there is no buildup left in corners or detail areas that could affect the quality of the finished cast.

4. Tear the fiberglass mat into 5-inch squares (adjust for project size). Cut the fiberglass cloth into squares using a pair of scissors.

5. Mix a batch of gel coat and catalyst in a disposable bucket. The exact ratio of gel coat to resin will be on the product container. The gel coat is thick, so completely mix in the catalyst. If it is not mixed completely, you will have gummy areas of partially cured fiberglass on the finished cast.

6. Brush a coat of catalyzed gel coat a quarter-inch thick into the inner surface of the mold. Make sure all nooks and crannies are completely filled, and pop any air bubbles you find. This will be the outer surface of your finished cast, so it must be as flawless as possible.

7. Allow the gel coat to fully cure until it is hard and no longer sticky to the touch.

8. Mix a batch of fiberglass resin and catalyst in a new bucket.

9. Brush a layer of catalyzed resin onto the gel coat.

10. Press squares of fiberglass mat onto the entire inner surface of the mold, overlapping them by a quarter-inch to ensure complete coverage. You will use mat for the first few outer layers because it is easier to sand than cloth.

11. Tap more resin into the layer of fiberglass mat with the brush, saturating it fully and forcing out any trapped air bubbles.

12. Immediately add another two layers of fiberglass mat and resin.

13. Add several alternating layers of fiberglass cloth and mat, using resin after each layer.

14. Allow the finished cast to fully cure.

15. Unscrew the mold and remove it from the cast.

16. Trim any rough edges from the fiberglass with a rotary tool.

17. Fill in any surface holes created by air bubbles with more gel coat. Sand them smooth once the gel coat has cured.

Tips & Warnings

• Always wear protective gloves when working with fiberglass and resin.

• Always wear goggles and a dust mask when cutting or sanding fiberglass.

References

• Serious Explorations: Step by Step Fiberglass
• "Car Stereo: Fiberglass"