How to Repair a Flaking Clear Coat on a Fiberglass Boat

by Gus Stephens
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The clear exterior coat on fiberglass boats is formed by a substance known as gelcoat. A two-part system consisting of polyester resin and catalyst, gelcoat's the first thing into the mold during manufacture. As it cures, gelcoat bonds to the layers of structural fiberglass laid on it to create a smooth, shiny exterior coat. Gelcoat has two functions in marine applications. It seals the fiberglass structure beneath it to protect it from the elements and it gives the boat a glossy, attractive appearance. Gelcoat is under continual assault from impacts, structural flexing and ultraviolet damage from the sun. It's more a matter of when than if repairs will be required to cracked or flaking gelcoat on a well used fiberglass boat.


Step 1

Chip away the cracked or flaking gelcoat with a sharp knife, expanding outward until you reach an area where it is intact and firmly bonded to the fiberglass beneath.

Step 2

Clean the chipped away area with alcohol or acetone solvent.

Step 3

Sand the layer of fiberglass exposed by the removed gelcoat with coarse 80- to 150-grit dry sandpaper to provide good bonding surface for the application of new gelcoat.

Look for a pattern of spider web-type cracks radiating away from the damaged area. Use a rotary grinder on the Dremel tool or a sharp knife to open up any cracks and enlarge them.

Applying Gelcoat

Step 1

Mix the two-part gelcoat paste and catalyst in a mixing cup according to manufacturer's instructions. Once the components are mixed, you have only 15 to 20 minutes of working time before the gelcoat begins to harden.

Step 2

Apply the gelcoat to the repair area. Several methods work, depending on the size and depth of the repair. You can paint mixed gelcoat on, preferably with a fine-bristle brush, or spread it across the surface with a razor blade or other thin, metal implement. You can dab it on to larger areas with a mixing stick and then smoothed with a plastic spatula. Build up the coating of new gelcoat so it is slightly elevated above the surrounding intact surface to leave some margin for sanding. Press the gelcoat into any enlarged cracks to fill them completely.

Smooth the surface of the repaired area by rolling a glass bottle over the surface. Cover the repaired surface with plastic food wrap or a piece of mylar taped down.


Step 1

Peel off the plastic wrap after the gelcoat has fully hardened.

Step 2

Sand finish the repair area beginning with 150-grit dry sandpaper on a sanding block. Sand in a back and forth motion. Continue until the repair area is sanded down to the level of the existing gelcoat.

Step 3

Continue to sand with successively finer grits of wet/dry sandpaper from 220-grit up to 600-grit to remove the scratches and finish the surface. Spray the surface with water from a spray bottle at intervals as you sand to smooth the finish.

Wipe the repair area dry. Coat the surface with rubbing compound and buff the surface by hand or using a power buffer. Apply wax formulated for fiberglass to the repair area. After it dries to a haze, buff the wax with by hand or using a power buffer.

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