Fiberglass Boat Repair: Materials, Tools, and Step-by-Step Instructions

Fiberglass repairs, like almost any project you can imagine, begins with preparation and spreads out from there. Fiberglass damage can penetrate the fiberglass skin of the boat or merely crack the surface. Whether it goes all the way through or not, water can infiltrate into the core material of the boat, hindering its flotation. Either type of damage requires repair at the earliest possible time. The materials, tools and the step-by-step instructions for this type of repair are straightforward.

Things You'll Need

  • Dish detergent and water
  • Clean cloths
  • Acetone
  • Disk grinder
  • 40-grit sandpaper
  • 1.5 ounce fiberglass mat
  • 6.0 ounce fiberglass cloth
  • Marine epoxy kit
  • Plastic putty applicator
  • 420-grit sanding block
  • Fiberglass wax
  • Clear plastic sheet
  • Duct tape
  • Gelcoat repair kit
  • 420-grit sandpaper
  • Palm sander
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Instructions

  1. Preparation

    • 1

      Clean the surface by scrubbing it with a mixture of dish detergent and water in the ratio of one part dish detergent and 20 parts water, using a scrub brush or deck brush. Then, to prevent the grinder you will use later from fouling on bits of wax on the surface, wash the damaged area with clean cloths saturated in acetone.

    • 2

      Grind the surface with a disk grinder and a 40-grit sanding disk, to increase the area to which the fiberglass and epoxy will adhere. If the damage is such that you must grind down into the fiberglass by 1/4 inch, multiply the depth to which you ground by 24. The result, 6 inches, is the diameter of the area you must grind to a gentle slope, sloping down to the bottom of the area you wish to repair.

    • 3

      Cut the fiberglass, both fiberglass cloth and fiberglass mat to fit the area, increasing in size until you have a mat that possesses the same diameter as the gently sloping area of ground fiberglass.

    • 4

      Mix the two parts of the marine epoxy in a clean container, using a wooden stirring stick such as you might find at a paint store. Follow the proportion guide and instructions on the side of the cans or the package in which the kit cans come.

    If the Damage Does Not Penetrate

    • 5

      Spread a layer of epoxy on the bottom of the hole, using a plastic putty applicator. Lay the smallest layer of 1 1/2-ounce fiberglass mat in place on the epoxy and press it into the epoxy.

    • 6

      Spread the second layer of epoxy over the fiberglass mat and beyond its edges. Lay the second layer of fiberglass -- use 6-ounce fiberglass cloth for the second layer -- and press it into the epoxy.

    • 7

      Add the two subsequent layers this way, then stop and allow these layers to cure fully before adding layers of fiberglass. Continue to add layers of fiberglass and epoxy until the epoxy is slightly higher than the deck.

    • 8

      Wait until the last layer cures fully, then sand the fiberglass down with a 420-grit sanding block, then apply fiberglass wax.

    Through-and-through Hole

    • 9

      Tape a sheet of clear plastic wrap over the hole, using duct tape. Make the plastic as taught as you can.

    • 10

      Move inside the cabin or hull, so that you are looking out the hole. Mix up the contents of a small gelcoat repair kit, and pack a layer of gelcoat into the hole, against the clear plastic.

    • 11

      Spread marine epoxy over the gelcoat gently, without mixing, and lay the smallest piece of 1.5-ounce fiberglass mat onto the epoxy. Gently press the mat into the epoxy as you spread a second layer of epoxy over the first. Press a piece of 6-ounce fiberglass cloth into it. Repeat two more times, then wait for the epoxy to cure.

    • 12

      Add additional coats of epoxy and fiberglass, if needed. Alternate between fiberglass mat and fiberglass cloth, and ensure that the last layer you put over the damaged area is epoxy.

    • 13

      Sand the surface of the repair with 420-grit sandpaper and a palm sander. Apply fiberglass wax according to the wax instructions. Remove the clear plastic sheet from the damaged area after 24 hours.