Fiberglass Repair

Few things are more disheartening to the boatowner than staring at the jagged edge of broken fiberglass. However, the repairability of fiberglass is one of its best characteristics. Even large holes in a fiberglass hull can be repaired with a bit of glass fabric, a supply of resin, and equal parts skill and care. And the repair is less patch than graft-a new piece of skin indistinguishable from the old.

Cut Away the Damage
Never try to save damaged fiberglass; always cut it out and replace it with new laminate. Check all the edges and enlarge the hole if you find any additional delamination.

Dewax
Before grinding, always wash the area around the hole thoroughly with a dewaxing solvent. If you fail to remove the wax first, grinding will drag it into the bottom of the scratches and weaken the bond.

Grind
Use a disk sander loaded with a 36-grit disk to grind a 12-to-1 bevel around the perimeter of the hole inside. Also grind an area of the inner surface a few inches beyond the bevel to accommodate a finishing layer of cloth.

Mask and Mold
Mask off the area around the exterior of the hole and tape heavy paper or plastic below the hole to prevent resin runs from adhering to the surface.

Fabric
Cut the first piece of fiberglass matt to fit the hole with a substantial overlap over the edge. Cut the next piece the size of the hole, and the succeeding pieces larger and larger to build the laminate up and out to fit the leveled edge. You may have to allow the first layers to set a little and stiffen before applying additional layers. Unless you have reason to follow a different schedule, begin with two layers of 1 1/2-ounce mat, then alternate mat and 6-ounce cloth.

Using Polyester or Vinylester Resin
For above-the-waterline repairs you can use either polyester or vinylester resin. Of course, for an even stronger repair you can also use epoxy, but not if the surface of the repair will be gelcoat. (You could use epoxy for underwater repairs.)

Catalyst
The catalyst for both polyester and vinylester resin is methyl ethyl ketone peroxide, or MEKP. Do not confuse MEKP with the common solvent MEK; they are not the same.

Gelcoat
Gelcoat is essentially pigmented polyester resin. Start the repair by spraying or brushing about 20 mils of color-matched gelcoat onto the waxed backer.

Finish
For a finished look, cut a rectangular piece of mat and one slightly larger of cloth and apply these over the patch, smoothing them with a squeegee. Seal this top layer with plastic or PVA to allow a full cure. Remove the backer from the exterior surface. Fill imperfections in the new gelcoat with gelcoat paste and allow it to cure fully. Clean the area around the patch, then sand-if necessary-and polish the repair area.