How to Repair A Fiberglass Hull On Boats

Written by anna roberts
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How to Repair A Fiberglass Hull On Boats
(Michal Zacharzewski

Rot, collisions and running aground, can all cause damage to a fibreglass boat hull. The problem can be quite severe if the damage is extensive or is left neglected. A damaged hull can lead to flooding and eventually sinking. If a hull has significant damage it should be hauled out of the water but if the damage is small and near the surface it can usually be repaired from the inside while docked.

Skill level:
Moderately Challenging

Things you need

  • Fibreglass repair kit (fabric, resin, catalyst, and gel coat)
  • Scissors
  • Sand paper
  • Disk Sander
  • Tape
  • Rags
  • Safety glasses
  • Respirator or dust mask
  • De-waxing solvent
  • Polyvinyl alcohol
  • Formica
  • Paste wax
  • Resin roller
  • Heat gun (optional)

Show MoreHide


  1. 1

    Cut and completely remove the damaged and surrounding area. Cut out a circle with a diameter 12 inches greater than the damaged area. For example, if the size of the damaged area is 4 inches, cut out an area with a diameter of 16 inches. Work from inside the boat, if possible.

  2. 2

    Use a de-waxing solvent to remove the wax from inside and outside of the hole.

  3. 3

    Sand the area around the hole with a disk grinder and sand paper, inside and out.

  4. 4

    Apply a generous coat of paste wax to the exterior of the repair area only, avoiding the edge.

  5. 5

    Cut out a piece of Formica a bit larger than the hole to be repaired. Apply polyvinyl alcohol to the Formica and tape it in place on the outer hull. This forms a backer. If you need to, you can use a heat gun to mould the Formica but it will most likely mould well by itself.

  6. 6

    Cut a piece of fibreglass fabric from your fibreglass repair kit, 1 inch larger than the hole. You will need to use several layers. The number of layers will be determined by the thickness of the hole but generally you will use two layers of 42.5gr mat followed by alternating mat with 170gr cloth. Each subsequent layer should be cut 1/2 inch larger than the previous.

  7. 7

    Follow the instructions in your fibreglass repair kit to apply resin and a catalyst. Epoxy is used for repairs below the water line but cannot be used with gel coat.

  8. 8

    Paint or spray the gel coat from the repair kit onto the Formica backing about 20mm thick, working from the inside.

  9. 9

    Wet the fabric and mat with polyester resin and apply the first three layers of mat and fabric to the gel coat, smoothing bubbles with a resin roller. Then apply the next four layers. Don't apply more than four layers at a time.

  10. 10

    Cut a final piece of mat 1 inch larger the repair area in a rectangular shape and apply it with resin and a resin roller to create a finished look. Seal the final layer with polyvinyl alcohol.

  11. 11

    Remove the Formica backing and fill imperfections with gel coat.

Tips and warnings

  • Clean your tools with acetone.
  • Sanding fibreglass is dangerous because the dust becomes airborne. Dress appropriately and wear safety gear so that the dust does not come into contact with your skin. Thoroughly clean up the dust after sanding.

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