How to Fill Old Screw Holes in Fiberglass Decks and Hulls

Back out screws, remove bolts, and drill out rivets. Scrape away old sealant around the hole with a plastic scraper. If the old hardware was set in a strong polyurethane adhesive, see /guides/deck-hardware-rebedding for how to free it without tearing the gelcoat.

Shine a light into the hole. Bare fiberglass all the way through means solid laminate. Wood, balsa or foam between two skins means a cored deck. Poke the core with a small pick: dark, mushy or wet core, or water that weeps out, means water has already gotten in. Press and tap the deck around the hole with a plastic mallet; a dull, dead sound or a soft feel suggests a wider wet area (see /guides/deck-core-moisture).

Never fill over damp core: the epoxy won't bond and the water stays trapped. Practical Sailor suggests warming the area with a hair dryer until no more moisture comes out. Small, slightly damp spots can be dried this way over a day or two. If the core is rotten or the wet area is bigger than a small patch around the hole, stop and plan a proper core repair or call a pro.

Epoxyworks describes the basic prep: drill the hole slightly larger than before to expose fresh, clean material, then chamfer the top edge with a countersink, and the underside too if you can reach it. The chamfer helps keep the repair from showing through the finish, and a chamfer on both sides locks the plug in place. Clean with acetone and let it dry. Surfaces must be clean, dry and sanded for epoxy to grip. Practical Boat Owner stresses that this oversize drilling is the quickest, surest way to get rid of old sealant inside the hole: any sealant left behind stops the epoxy from bonding to that spot.

For a cored deck, BoatUS describes the overdrill method: drill the hole oversize, then chuck a bent nail into the drill and spin it inside the hole to dig out the core about 1/4 inch around the hole under the top skin. Vacuum out the debris. This leaves a cavity that the epoxy turns into a solid, sealed plug, so later fasteners never touch the wood. Practical Boat Owner points out a second benefit: the solid plug stops the deck sandwich from being crushed when you tighten bolts through it later. An Allen (hex) key in the drill works the same way on balsa; foam core is tougher and may need a small chisel.

If the hole goes all the way through and you can reach the underside, cover it with duct tape so the epoxy can't run out. On a blind hole you can't reach behind, Epoxyworks suggests pushing a small epoxy-wetted plug of absorbent material to the back first so the fill doesn't run through.

Practical Sailor's tests found fast hardeners and five-minute epoxies build up heat in a hole this size and cure full of bubbles, making a weak plug. Use a slow hardener and mix exactly as the label says. They also advise filling when the temperature is steady or falling: on a warming day, air expanding out of the core bubbles up through the fill.

Fill the hole with unthickened epoxy first so it soaks into the exposed core and the fiberglass edges. BoatUS describes letting it sit a minute or two to saturate the core, then puncturing the tape and draining the excess back into the cup. On vertical surfaces, inject it with a syringe instead.

• Simple fill you won't drill again: thicken epoxy with high-density filler or colloidal silica to a peanut-butter or mayonnaise consistency (stiffer on vertical surfaces) and fill the hole. Epoxyworks recommends injecting from the bottom of a blind hole with a syringe, because pushing filler in from the top with a knife traps an air bubble at the bottom.
• A screw will go back into this spot: Practical Sailor's tests found that fillers create tiny air bubbles that weaken threads, while plain epoxy, or plain epoxy packed with small strips of light fiberglass cloth, held screw threads far better. Fill with plain epoxy, push strips of cloth in with a blunt stick, work out the bubbles and top it off. Micro-balloons are for fairing, not strength.
• Fill level: for a gelcoat finish, stop the epoxy a little below the surface to leave room for gelcoat (step 11). For a painted finish, overfill slightly and sand flush later.

Let the epoxy cure fully, as long as the label says for the temperature. If you're mounting hardware again, BoatUS says to redrill the mounting holes through the cured epoxy and then bed the hardware with sealant as usual (see /guides/deck-hardware-rebedding). Drill the correct pilot size so screws don't bind or crack the plug. For anything that takes real load, a through-bolt with a backing plate is better than a screw (see /guides/mounting-accessories-through-bolt).

Wash the cured epoxy with water to remove the waxy film (amine blush) some epoxies leave, then sand it lightly. Epoxyworks reports that polyester gelcoat holds well over properly prepared, fully cured epoxy above the waterline. Apply color-matched gelcoat paste slightly proud of the surface, let it cure, then wet-sand flush with progressively finer paper and polish (see /guides/gelcoat-repair). For a good color match, Practical Boat Owner suggests first sanding and polishing a small patch of the deck next to the hole, so you match the true color of the molding rather than faded, chalky gelcoat. It also notes that even pros can spend an hour getting a match right, so if a shop ever does gelcoat work on your boat, ask them to mix a little extra for you to keep. Don't leave bare epoxy exposed: Practical Sailor warns it breaks down in sunlight within months.

An old transducer, drain or through-hull hole below the waterline has the whole weight of the water pushing on it. Epoxyworks rates hole repairs by hole size, laminate thickness and the consequences of failure, and advises erring on the conservative side. Larger or thinner-skinned holes need the edges ground to a long, gentle bevel and new fiberglass layers built up to the original thickness, not just a plug. BoatUS notes that leaks at fittings bolted through the hull generally mean hauling the boat for a proper repair. Unless the hole is tiny and you have glass-repair experience, have a yard or fiberglass shop do it while the boat is out, and never launch until the repair has fully cured.

After the first heavy rain and the first hard wash-down, look at the repairs from below where you can. A well-done fill stays dry and invisible. Note each filled hole, its location and what you used in your BoatMend log so a future owner or surveyor knows the core was sealed.