How to Patch a Fiberglass Hull
A holed fiberglass sailboat or canoe looks bad, but unless the boat is structurally damaged, it's easy to repair. With patience, you can rebuild the hull as good as new.
Tools: disc sander or electric drill with sanding disc, buffing attachment for sander or drill, scissors, mixing can and stir stick, paintbrushes, sanding block.
Materials: medium-grit sanding discs, soft cloths, acetone, light cardboard, plastic wrap, masking tape; quart-size polyester resin fiberglass repair kit, or polyester resin and hardener, 10-ounce roving, and 7 1/2-ounce fiberglass cloth; fine, very fine, and extra-fine wet-or-dry sandpaper; gel coat to match hull, fiberglass rubbing compound, car wax.
Time: about 2 days, depending on size of hole.
To patch the hole, buy a quart-size polyester resin fiberglass repair kit at a marine store; from the boat's manufacturer, buy gel coat to match the color of your boat. If you can't find a repair Kit buy 1 quart of polyester resin and hardener, and 2 square feet each of 10-ounce roving and 7 1/2-ounce fiberglass cloth. Buy fiberglass rubbing compound to finish the repair.
To prepare a firm base for the patch, enlarge the hole. Grind down the edges of the hole to firm fiberglass with a disc sander or an electric drill with a sanding attachment, and a medium-grit sanding disc. When you've exposed sound fiberglass all around the hole, taper the ground hole out into the sound hull, so that the outside edge of the hole is at least 2 or 3 inches wider than the inside edge. This provides a firm bonding surface for the patching material.
When the hole is completely sanded, wipe the dust off with a soft cloth and then clean the damaged area with a cloth soaked in acetone. Caution: Acetone is extremely flammable. Work in a well-ventilated area, and don't smoke; dispose of acetone-saturated cloths immediately in an outside vented container.
To contour the patch to match the shape of the hull, cut a backing piece from light cardboard. Cut the cardboard at least 2 inches larger than the hole all around. Cover the piece of cardboard smoothly with plastic wrap, taped into place, so that the patch won't stick to it, and then set the backing into place on the inside of the hull; it should cover the hole completely. Carefully bend the cardboard to match the curve of the hull, and tape it firmly into place with masking tape. The curve of the cardboard must match the hull contour precisely or the patch will be weak.
When the hole is backed, prepare the patching material. The hole will be filled with pieces of 10-ounce roving to the thickness of the hull; the top layer is cut from 7 1/2-ounce fiberglass cloth to match the hull surface.
Cut the first layer of roving to fit the inside diameter of the hole, at its smallest point. Cut successive layers slightly larger, to overlap the inside layers and feather out over the tapering edges of the hole. Stack the layers of roving in the hole as you cut them so you can see how many thicknesses to cut. When you have enough layers of roving to make the depression just level with the surrounding hull surface, cut a piece of fiberglass cloth to complete the patch, overlapping well onto the sound hull surface.
Lay the pieces of roving and fiberglass out in order on a clean surface near the boat. Then, with your patching material at hand, mix polyester resin and hardener in a can as directed by the manufacturer. Mix only as much resin as you can apply in less than 1/2 hour; it hardens quickly. Make sure the temperature in your work area is at least 60° F.
Set the first patch of roving -- the smallest one -- on a clean piece of cardboard near the boat. Working quickly, apply catalyzed resin to the roving with a small paintbrush; brush the resin well into the roving to soak it thoroughly. The roving should be completely wet through, but not dripping. Pick up the piece of wet roving and set it into the prepared hole in the hull, centered in the bottom of the depression. Press it smoothly into place with the brush, dabbing it from the center toward the edges.
Set the next smallest patch of roving on the cardboard and apply catalyzed resin to it as above; soak it thoroughly. Position the saturated roving over the first layer in the hole, and smooth it into place with the brush. Repeat, applying resin and smoothing each roving layer into place, to build up a smoothly graduated patch of new material in the hole, tapering with the edges of the hole to match the contour of the hull.
When all the layers of roving have been applied, close the patch with the prepared piece of fiberglass cloth. Instead of soaking the cloth with resin, set it directly over the patch, dry, and work it down into the resin-soaked roving with the paintbrush. If necessary, add a small amount of additional resin to saturate the edges of the fiberglass cloth, but be sparing; too much resin will weaken the patch. Use only enough resin to thoroughly wet and darken all the roving and cloth.
Let the completed patch dry for at least 8 hours, as specified by the resin manufacturer. When the patch is completely dry, remove the cardboard backing from the inside of the hull; the patch should follow the contour of the hull exactly. Remove any irregularities in the patch surface, and correct contour as necessary, with a disc sander or an electric drill with a sanding attachment, and a medium-grit sanding disc; do not oversand, but smooth the area to match the hull contour exactly. Then smooth the sanded surface by hand with a sanding block and fine-grit wet-or-dry sandpaper, using long back-and-forth strokes over the patch.
When the patch is smooth and matches the hull contour, clean the area with acetone to remove all dust. Finish the patch with gel coat, to match the color of the hull. Mix the gel coat as directed by the manufacturer. Brush a thin layer of gel coat onto the patch area, to cover the patch completely and evenly. Then, working quickly, carefully smooth a large piece of plastic wrap directly over the wet gel coat, smoothing out all air bubbles and wrinkles, to cover the patch completely. If necessary, use overlapping strips of plastic to cover the patch completely. You must cover the gel coat with plastic or it won't dry smooth.
Let the gel coat dry for at least 8 hours, as directed by the manufacturer. When the gel coat is completely dry, peel off the plastic wrap; the patch surface should be very smooth. Carefully hand-sand the patch area with fine-grit wet-or-dry sandpaper to remove any excess gel coat and contour the patch to match the hull exactly. Smooth the sanded patch carefully and remove any scratches with very fine and then extra-fine wet-or-dry sandpaper; use enough water to prevent scratching the gel coat.
Finally, clean the rebuilt hull area thoroughly with a wet cloth. To polish the patch, apply fiberglass rubbing compound to the sanded gel coat and buff the surface as directed by the manufacturer; use the buffing attachment of the disc sander or electric drill. After buffing the patch, apply a protective coat of car wax to the hull surface.
Now that your boat's hull is patched and buffed, let's turn our attention to the interior of the boat. Learn how to make built-in waterproof cockpit cushions on the next page.
For tips on caring for and repairing other types of sports equipment, try the following links: