A chip in a fiberglass hull is more than a cosmetic annoyance. Gelcoat is only about half a millimeter thick (roughly 20 mils), so a hard knock from a dock piling, an anchor fluke or a careless dinghy can go straight through it and expose the laminate underneath. Once water can reach the glass fibers, it wicks in, and over time that can mean blisters, stains and a much bigger repair.

Quick answer: clean and de-wax the area, grind the chip open into a shallow, bevelled bowl, then fill it. If the chip stops in the gelcoat, fill it with catalyzed gelcoat paste. If it reaches the laminate (or it is below the waterline), fill it with thickened epoxy first and finish with gelcoat or paint. Seal gelcoat from the air while it cures, then wet-sand from 320 to 1,500 grit, compound, polish and wax.

This guide covers impact damage: chips, dings, gouges and small holes. If you are dealing with long, linear scratches that haven’t broken through the gelcoat, the approach is a little different, and we cover it in our guide on how to repair deep scratches in gelcoat.

Step 1: Assess How Deep the Damage Goes

Before you buy materials, figure out what you are actually repairing. Clean the area and look closely, ideally with a magnifier and a flashlight held at a low angle.

• Gelcoat only: the bottom of the chip is still colored gelcoat, with no visible fibers. This is a purely cosmetic fix.
• Into the laminate: you can see white or translucent glass strands or the darker resin-rich layer below the gelcoat. The repair now needs a structural filler before the finish.
• Cracked or soft laminate: spider cracks radiating from the impact, flexing when you press, or a dull sound when you tap around it with a plastic mallet or screwdriver handle. That points to delamination or, on a cored hull or deck, a damaged or wet core.
• Hole through the hull: anything you can see daylight or the inside of the boat through is a structural repair, not a chip repair.

Also check the inside of the hull behind the chip if you can reach it. Hairline cracks on the inner skin mean the impact did more than chip the surface. Cracks that keep reappearing in the same place, especially near hardware, bulkheads or hull-to-deck corners, usually come from flexing rather than a one-off impact, and they need reinforcement, not just filler. Our article on common sailboat problems and how to fix them covers some of these structural warning signs.

Damage	Typical cause	Repair material	DIY?
Chip in gelcoat only	Light knock, dropped winch handle	Gelcoat paste	Yes
Chip or gouge into laminate, above waterline	Docking impact, anchor	Thickened epoxy, then gelcoat or paint	Yes, with care
Chip or gouge below waterline	Grounding, debris, trailer	Thickened epoxy, barrier coat, antifouling	Yes, with care
Star cracks, soft spots, wet core	Hard impact, flexing	Grind out, laminate, possibly core repair	Experienced DIY or pro
Hole through the hull	Collision, grounding	Structural laminate repair	Usually a pro

Tools and Materials

For most chip repairs you will need:

• Acetone or a dedicated de-waxer/solvent wipe, and clean lint-free rags
• A rotary tool (such as a Dremel) with a small sanding drum or carbide burr, or a sharpened can opener for small chips
• Masking tape and mixing cups, sticks and a small flexible plastic spreader
• Gelcoat paste or gelcoat with wax additive, MEKP hardener and color tints (or a matched repair kit)
• Epoxy resin and hardener plus a thickener such as colloidal silica, for chips that reach the laminate
• PVA mold release or plastic film to seal air-inhibited gelcoat while it cures
• Wet-or-dry sandpaper in 220, 320, 400, 600, 800, 1,000 and 1,500 grit, and a small sanding block
• Rubbing compound, polish and a marine wax
• Nitrile gloves, safety glasses and a respirator rated for organic vapors and dust

Step 2: Clean and Open Up the Chip

1. Wash the area with soapy water, rinse and let it dry. If the boat has just been hauled, see how to pressure wash a boat hull without lifting loose gelcoat around the damage.
2. Wipe it down with acetone or a de-waxer, using one rag to wet and a clean one to wipe off before the solvent evaporates. Wax and silicone are the main reason patches fall out, so don’t skip this.
3. Remove every loose or cracked flake of gelcoat around the chip. Tap gently around the edges: anything that sounds hollow has lost its bond and has to go.
4. Grind the chip into a shallow, bowl-shaped depression with bevelled edges. You want sound material on all sides and a gentle slope rather than a vertical edge, which gives the filler more surface to grip and makes the edge easier to blend.
5. Lightly scuff about half an inch (12 mm) of the surrounding gelcoat with 220 grit so the patch has something to key into, then mask just outside that area.
6. Vacuum or blow out the dust and wipe again with acetone.

For a structural repair through several layers of laminate, the bevel needs to be much longer. Epoxy manufacturers commonly recommend a taper of about 12:1 (twelve units of bevel length for every unit of laminate thickness), which is beyond a simple chip repair and the point where many owners call a professional.

Step 3: Fill the Chip

Gelcoat-only chips: gelcoat paste

Gelcoat paste is thicker than brushing gelcoat and won’t run out of a vertical repair. Mix only as much as you will use in a few minutes, add color tint first, and stir in MEKP hardener last, following the manufacturer’s ratio (often around 1 to 2 percent by volume, but check the can). Stir thoroughly: uncatalyzed streaks will never cure.

Press the paste into the chip with the spreader, working out air pockets, and leave it slightly proud of the surface. Polyester gelcoat shrinks a little as it cures, and you will be sanding it flush anyway.

Most repair gelcoat is air-inhibited: the surface stays tacky as long as it is exposed to air. Either use gelcoat that contains a wax additive (sometimes called surfacing agent or sanding aid), spray it with PVA, or tape a piece of plastic film or wax paper smoothly over the patch until it cures.

Chips that reach the laminate: epoxy first

When the damage goes into the glass, fill it with epoxy thickened with colloidal silica to a peanut-butter consistency. Epoxy bonds far better to already-cured polyester laminate than polyester resin does, shrinks very little and is a better moisture barrier. That makes it the safer choice for a secondary bond like this.

• Wet out the bare laminate first with a thin coat of unthickened epoxy, then press the thickened mixture in while the first coat is still tacky.
• If you plan to finish with gelcoat, stop the epoxy roughly 1/32 inch (about 1 mm) below the surface to leave room for it.
• Let the epoxy cure fully, wash off any amine blush with water and a scrub pad, then sand the surface before applying gelcoat. West System has tested polyester gelcoat over properly prepared, fully cured epoxy and found it bonds well, though some gelcoat makers still advise against it, so always follow the label of the product you use.
• An alternative is to fair the epoxy flush and finish with a color-matched marine topside paint instead of gelcoat.

Chips below the waterline

Under the antifouling, nobody sees the color, so there is no need for gelcoat at all. Fill with thickened epoxy, fair it flush, apply a few coats of epoxy barrier coat over the patch, then repaint with antifouling. Let the boat dry out well first: epoxy won’t bond properly to wet laminate. If the laminate around the chip stays damp or you find blisters, read our guide on buying or owning a boat with osmosis before you seal anything in. For bottom paint options, see the best antifouling bottom paint for boats.

Step 4: Match the Color

Color matching is the hardest part of a cosmetic repair. A few tips help a lot:

• Compound and polish the area around the repair first. Oxidized gelcoat looks paler and chalkier than its true color.
• Start with a white or neutral base and add tint in tiny amounts. Old white hulls are rarely pure white; most have yellowed or greyed slightly.
• Make small test dabs on a card, catalyze them, let them cure and compare in daylight. Gelcoat often changes shade as it cures.
• When you have a match, write down the ratio and mix the final batch exactly the same way.

If you are going to repaint a large area anyway, matching matters less. Our guide to painting over gelcoat explains when paint is the better long-term finish.

Step 5: Sand, Compound and Polish

Once the patch has cured hard (typically overnight in warm weather; much longer in the cold, and many gelcoats won’t cure properly below about 60°F/15°C), remove the film or wash off the PVA.

1. Knock down the high spot with 320 grit on a small, hard block, sanding only the patch. Use the block so you don’t dish the surrounding surface.
2. Switch to wet sanding and work up through 400, 600, 800, 1,000 and 1,500 grit, keeping the paper and surface wet. Feather the edges until you can’t feel the transition with your fingertips. On curved surfaces, follow the curve rather than sanding flat.
3. Wipe dry and inspect. Pinholes or low spots mean another thin skim of paste and a repeat of the sanding.
4. Machine or hand compound the area until the sanding haze is gone, then polish.
5. Finish with a coat of marine wax or sealant. Our roundup of boat coatings, waxes and sealants covers the options.

Safety Notes

• Wear eye protection when handling MEKP: it is corrosive and especially dangerous to eyes.
• Grinding fiberglass produces fine, itchy dust. Wear a properly fitted respirator, long sleeves and gloves, and vacuum as you go.
• Gelcoat and polyester resins release styrene; epoxy can cause skin sensitization with repeated contact. Work with good ventilation and never handle resins with bare hands.
• Dispose of rags soaked in solvent or catalyst safely; mixed resin can get hot as it cures.

When to Call a Professional

Small chips are a very achievable DIY job. Call a boatyard if the damage is through the hull, if you find a soft or wet core, if cracks keep coming back, or if the chip is next to a through-hull, keel joint or rudder. A surveyor can also tell you whether the impact damaged anything you can’t see; our list of hidden problems to look for in used sailboats explains what they check.

Summary

Repairing a chip in a fiberglass hull comes down to five steps: assess the depth, clean and open the chip, fill it with the right material, match the color and sand and polish it back to a shine. Use gelcoat paste for shallow chips, thickened epoxy for anything that reaches the laminate or sits below the waterline, and always seal air-inhibited gelcoat while it cures. Done carefully, a small repair is nearly invisible and keeps water out of the laminate for years.