Things You'll Need
1-inch hole saw
Automotive paste wax
Mold release spray
Gel Coat resin and hardener
Epoxy resin and hardener
Small paint roller
Common building studs
When you notice a small soft spot in the deck of your boat, it's often easy to ignore it, thinking the problem is one of limited scope. The problem, though, is that soft spots indicate an underlying problem that, if ignored, will spread. Repairing a soft spot in a fiberglass deck is both a monstrous dance with reciprocating saws and grinders that cut into your boat, and a precise ballet of blending chemicals, cutting materials, and applying them to the wound to produce a finished product barely distinguishable from the original.
Tap the area around the soft spot with the butt-end of a screwdriver--a sharp sound is good, a hollow thud indicates a weak area. Mark the weak areas around the soft spot with a grease pencil, and include them in the perimeter of the hole you're going to cut in your deck. When you've located all the weak areas and soft spots, use a drill and a 1-inch hole saw to make a hole into which you can insert the tip of a reciprocating saw. Use the saw to cut the soft spot and weak areas out of your boat's life.
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Apply a dewaxing agent such as acetone to the inside surface around the area you cut out. The limits should extend outward from the hole, 12 inches for each inch of deck thickness. Use a hand grinder to grind a 12-to-one slope into the inside surface of the area you cut out, so that for every inch of the thickness of the deck, you have 12 inches of slope.
Wax the surface around the outside of the hole with an automotive paste wax. Cut a sheet of countertop material that's a foot longer and wider than the hole, and paste-wax one side. Spray the paste-waxed side with mold-release spray (polyvinyl alcohol) and duct tape it over the hole, with the waxed side to the deck.
Pre-cut the fiberglass matting and cloth you'll use to fill the hole, using scissors. Don Casey, one of the boating experts for the Boat Owners Association of the United States, recommends beginning with "two layers of 1 1/2-ounce mat," then alternating the mat with 6-ounce fiberglass cloth. You'll need one layer of fiberglass for each 1/32-inch of deck thickness, with each layer of fiberglass cloth and mat 1/2 inch wider and longer than the previous layer.
Color match the gel coat to your boat, mix the hardener into the gel coat resin -- too little hardener is better than too much; if you err in the mix, err on the side of caution. Working from beneath the hole, inside the boat, use a paint brush to spread a layer of gel coat that's about the thickness of a toothpick onto the waxed counter top backer at the top of the hole. As you wait for the gel coat to harden, mix the epoxy resin and hardener for the fiberglass. When it starts to harden, begin laying up the fiberglass and epoxy resin, three layers at a time, until the repair is complete, pressing the fiberglass into the epoxy resin with a small paint roller or a squeegee.
Wait at least 24 hours for the gel coat to cure before removing the sheet of countertop material. Allow the fiberglass to cure thoroughly before walking on the repair.
Before you can begin to repair a large area that encompasses damage across multiple stringers, like the floor joists in your home, you'll need to replace the stringers you've removed. Use screws and a screwdriver to add "sisters" to the remaining stringer: screw a common building stud, cut to length, to each side of each end of the stringer you cut, so that these sisters cross the hole you cut out. Then make repairs as directed.
This project uses a variety of power and hand tools, and a variety of chemicals. Observe all precautions indicated for power tools and work in a well-ventilated environment.