Building a kayak from fibreglass is a challenging project with many advantages over building one in other materials. Fibreglass is often stiffer, easier to work with, and produces a kayak that is durable and easy to repair. Once you have the initial work of building a mould out of the way, subsequent boats are faster to build.
Select a design
Select a kayak design. Almost any commercial plan can be selected and used as long as you account for the difference in the hull thickness between the material called for in the plan and the fibreglass. Instead of using an existing plan, design a new boat using kayak design software. Both Kayak Foundry and Delftship are easy-to-use programs that make designing kayaks easy.
Find or build a plug. A plug is a part that looks just like the kayak design that you're building. To make a plug from plans, you build a kayak, usually out of wood, finishing just the outside. But an existing kayak could be used as a plug as long as you obtain permission from the kayak's designer and manufacturer. Once the plug is finished and the surface is perfectly smooth, cover it with paste wax and a PVA mould release. The better the finished surface of the plug, the better the final DIY fibreglass kayak will be.
Build a mould. A fibreglass mould for a kayak is usually a female part that you'll build the kayak into. A kayak typically has two moulds; one for the hull and one for the deck. Around the plug, build a 7.5 to 10 cm (3 to 5 inch) flange where the seam between the deck and hull will be. This flange provides a place to run a knife, producing a hull or deck that lines up exactly with the other side. To build the mould, cover each side of the plug with fibreglass. Using matt fibreglass is typically easier, because it conforms to the plug easier than some fabrics. Once dry, pull the plug from your mould and then sand and fill any imperfections in the mould. The mould is then prepped with multiple layers of wax and PVA.
Spray the mould with coloured gelcoat. Gelcoat provides a smooth finish to the outside of the kayak. Consider using a bright colour; it will be more visible on the water. For the hull consider using white, which hides scratches.
Lay fibreglass into the mould. The goal of the layup is to use just enough fibreglass to provide the desired stiffness and durability to the finished kayak. The more fibreglass used, the more durable and stiff the kayak will be, but more fibreglass means a heavier kayak. Using extra partial layers of glass in the sections of the kayak that will be exposed to more wear and tear is a good way to gain durability and save weight. Once the fibreglass is positioned correctly, wet out the glass using epoxy, vinylester or polyester. Epoxy is the strongest and most expensive, polyester the weakest and least expensive. Vinylester is a good mix of cost and characteristics. Once the fibreglass dries remove it from the mould. If you prepped the mould correctly, this should be easy.
Join the deck to the hull using fibreglass tape on the inside and outside. When fiberglassing over cured glass, rough the surface up with sandpaper before applying the new layer. This fibreglass seam holds the boat together, so make sure it reaches into the far ends of the kayak. Then install the remaining outfitting. Often it's easier to build a separate mould for the cockpit coaming and glue it to the kayak at this point. The bulkheads, foot braces and hatch covers should all be installed with epoxy or a marine sealant.