How to make a fiberglass boat mold

Building a boat mould is one of the first steps in building a fibreglass 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 fibreglass. Prepare the plug and then laminate a fibreglass shell around the plug. When dry, pop out the plug and finish the inside surface of the mould. Lay up the boat on the inside of the mould.

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 mould the deck first, apply the foam to the hull. Cover the foam with PVC tape, which stops resin from penetrating. This acts as the mould's flange.

Cover the plug's deck with a light coating of sprayed PVA mould 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 millimetre coat.

Spray a 0.5 millimetre thick layer of tooling gelcoat over the PVA mould 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 fibreglass mat against it and the gelcoat.

Lay 170gr fibreglass 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 fibreglass mat over the fibreglass 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 fibreglass process, you created a new flange out of glass.

Repeat everything, but the adding of a mini-cell flange to create a mould of the hull.

Separate the hull from the deck mould and pop the plug out.

Stabilise the outside of the mould by reinforcing it with plywood cut to match the mould's curves. Every couple of feet is enough to stabilise it. At each plywood location, reinforce the mould by applying more fibreglass mat. Cover the plywood edges with foam to help protect the mould.


Wear a respirator when spraying and when working with resin.

Things You'll Need

  • Prepared boat plug
  • Hot glue and gun
  • Minicel foam
  • PVC tape
  • Spray gun
  • PVA mould release
  • Tooling gelcoat
  • Cup gun
  • Fibreglass Mat
  • Fibreglass cloth, 170gr.
  • Polyester resin
  • Paint brush
  • Plywood
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About the Author

Bryan Hansel is a freelance photographer and kayaking guide who began writing in 1993. His outdoors articles appear on various websites. Hansel holds a Bachelor of Arts in English and religion from the University of Iowa.