Excerpted from Fiberglass Boat Repair Illustrated
On some early fiberglass boats you may encounter crazed gelcoat – that is, gelcoat that has hardened and become covered with a random network of small cracks. These cracks are not only unsightly, they can allow moisture into the underlying laminate. This is especially problematic for a cored hull or deck, where moisture can easily disrupt the core-skin bond, but even a single-skin hull can suffer blistering or internal delamination from invading moisture.
To repair crazing, first sand the crazed area with 100-grit sandpaper, then wipe down the surface with a solvent such as Interlux’s 2333N Reducing Solvent or DuPont’s Prep-Sol. Do not use acetone for this, as it is fast-drying and may not keep wax and grease in suspension long enough for you to wipe them off. Repair any dents, gouges, or other flaws with a thickened epoxy putty as described above, sand again (this time progressing to 150- to 220-grit) to ensure a smooth hull, and wipe down with solvent again. Follow this by undercoating with a primer. If you are using a two-part linear polyurethane paint (LPU) you will want to use the primer recommended by the maker, usually a two-part primer. If you are going to paint with a one-part topcoat, however, a one-part primer is appropriate. For best compatibility, you should use a primer and topcoat from the same manufacturer. Sand the primer coat with 220- to 320-grit if you think you will need two layers of topcoat, and/or 320- and then 400-grit prior to the final topcoat. You can either roll and tip the topcoat or have a professional spray it for you.
A severe case of crazing may require you to grind away the crazed gelcoat altogether, in which case you will need to replace it with new gelcoat (vinylester as opposed to polyester for underwater portions of the hull) or with an epoxy fairing compound, which can be troweled on much like spackling compound in a home. Another option is to make your own fairing compound by mixing a micro-ballon thickener into an epoxy or polyester resin. Boatyard workers use battens and specialized techniques to fair a hull, but you can achieve good results on your own by fairing a section at a time and building up the fairing in layers no more than 1/8 inch (3 mm) thick. After the last layer of fairing compound has set, the hull should be longboarded to ensure that it is fair and smooth.
For more tips and techniques to help you maintain and repair your fiberglass boat, purchase Fiberglass Boat Repairs Illustrated from Practical Sailor.
