Building a boat mold is one of the first steps in building a fiberglass boat. In short, the process involves building a plug, which is an exact duplicate of the boat you want to build. For small boats, like canoes and kayaks, the plug is often made from wood covered with fiberglass. Prepare the plug and then laminate a fiberglass shell around the plug. When dry, pop out the plug and finish the inside surface of the mold. Lay up the boat on the inside of the mold.
Use hot glue and adhere the mini-cell foam to the prepared plug along the parting plane of the boat. The parting plane is the point where the hull and deck meets. To mold the deck first, apply the foam to the hull. Cover the foam with PVC tape, which stops resin from penetrating. This acts as the mold's flange.
Cover the plug's deck with a light coating of sprayed PVA mold release. Cover the entire deck with two coats by first applying a light coat followed by a heavier second coat. The goal is to cover the deck with a 0.005 to 0.010 millimeter coat.
Spray a 0.5 millimeter thick layer of tooling gelcoat over the PVA mold release. Tooling gelcoat requires a special spray gun. John Sweet of Sweet Composites, reports a cap gun works well and requires minimal cleaning. Allow to dry according to the gelcoat's instructions. Spray a second coat of gelcoat if desired.
Reinforce the corner between the mini-cell foam and the deck by laying strips of fiberglass mat against it and the gelcoat.
Lay 6-ounce fiberglass cloth over the plug's deck and wet it by carefully brushing on polyester resin. Mix the resin according to the manufacturer's instructions. The glass must conform to all the plug's curves. Make sure to cover the flange with this glass. Allow the resin to cure.
Place three oz. of fiberglass mat over the fiberglass cloth and wet it with polyester resin. Reinforce the flange and other areas that might flex with extra glass. Use five oz. of mat as the reinforcement. Allow to dry.
Flip the plug over and remove the mini-cell foam. During the fiberglass process, you created a new flange out of glass.
Repeat everything, but the adding of a mini-cell flange to create a mold of the hull.
Separate the hull from the deck mold and pop the plug out.
Stabilize the outside of the mold by reinforcing it with plywood cut to match the mold's curves. Every couple of feet is enough to stabilize it. At each plywood location, reinforce the mold by applying more fiberglass mat. Cover the plywood edges with foam to help protect the mold.
Items you will need
- Prepared boat plug
- Hot glue and gun
- Minicel foam
- PVC tape
- Spray gun
- PVA mold release
- Tooling gelcoat
- Cup gun
- Fiberglass Mat
- Fiberglass cloth, 6 oz.
- Polyester resin
- Paint brush
- Boatbuilder's Manual; Charles Walbridge; 1986
- Sweet Composites: Mold Construction
- boat image by pearlguy from Fotolia.com