Few things are more disheartening to the boat owner than staring at the jagged edge of broken fiberglass. However, the repair-ability 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. The best part is that a properly done repair can be as strong as (or stronger than) the original.
West System, which has been a leader in this field for many years, offers a huge amount of information online including detailed guides for many types of fiberglass work. Another good resource is found at MAS Epoxies and in particular, their how to videos. Following are some general tips. Your job may require very different steps and you should always carefully follow product instructions.
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.
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. When grinding and sanding it is very important to wear goggles that totally enclose the eyes and OSHA approved breathing mask. You may also need a breathing mask for fumes as well as dust, depending on the job.
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.
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. Polyester is less expensive than vinylester and readily available but is more susceptible to osmosis, even above the waterline. Of course, for an even stronger repair you can also use epoxy, but if the surface of the repair will be gelcoat you'll have to spend more time preparing, sanding and dewaxing the resin to achieve adherence.
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 is essentially pigmented resin. It both protects the fiberglass underneath but primarily provides a cosmetic covering. This is a point where you must very carefully follow product instructions.
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.