To repair a fiberglass boat, cut a hole in the hull around the damaged area. Bevel the edges of the hole with a grinder, and remove the wax from the area around the hole. Grind the inside surface of the hull around the hole to create a slight depression. Coat the edge with resin, and fill the hole from the inside with layers of resin-saturated fiberglass cloth. Add filler as necessary, sand the interior and exterior, and apply a gelcoat.
When beveling the hole, the inside edge of the hole should be wider than the outside edge of the hole. Grind the depression around the hole on the inside, extending it approximately 2 inches from the beveled edge. Tape off the area around the hole on the exterior of the hull.
Cut the first piece of fiberglass cloth to cover the hole and the depression. Apply epoxy resin to the beveled edge and depression, wet the cloth, and spread it over the hole. Allow the cloth to harden slightly before applying additional layers. Each subsequent layer should be slightly smaller to fill in the depression and the hole.
Allow the fiberglass to cure completely. Apply a fairing mixture to the inside and outside to fill in any small dents or holes. Sand the hull smooth, making sure to follow the curve of the boat. Spray on pigmented gelcoat on the exterior of the hull according to the gelcoat manufacturer's instructions.