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Repairing gelcoat on any part of a fiberglass boat; hull, bottom, topsides, or deck comes down to six disciplined steps: inspect the damage, clean and taper the edges, mix and color match fresh catalyzed gelcoat, fill the void slightly proud of the surface, give it a full overnight cure under an airtight film, then wet, sand, and buff it back to a mirror-gloss.
Whether you are fixing a fingernail-deep scratch or a gouge the size of a coin, following these steps (and the safety rules that go with them) is the proven way to restore the waterproof, UV-resistant shell that keeps your boat’s laminate dry and strong.
Gelcoat is More Than Just Looks
Gelcoat is a pigmented polyester skin that is only 0.25–1 mm thick. It seals out water, shields the laminate from UV, and provides a showroom shine. Surveys of professional repair yards say up to 60% of all hull damages involve the thin layer. Ignore a chip and moisture will creep in, leading to blisters, delamination, or freeze cracks that cost far more than a weekend’s worth of work.
Understand the damage before you pick up sandpaper
Scratches, gouges, cracks or blisters?
A scratch is cosmetic, a blister or star crack hints at water trapped beneath the skin. Untreated moisture-blisters balloon and can undermine laminate strength.
Tap around the spot in question; a dull thud means the core is wet and may need more extensive work before any gelcoat goes on.
When to call a pro
If the damage extends through the cloth, spans a structural corner, or the hull shows spider-web cracking over a large area, bring the job to a repair shop. Gelcoat is strong but brittle; the laminate below provides the real muscle.
The complete gelcoat repair process
Safety Gear
- Respirator
- Safety goggles
- Gloves
Tools & Equipment
- Rotary tool with burr bits
- Sanding papers (80 to 1000-grit)
- Mixing cups & stir sticks
- Painter’s tape
- Polisher
Materials
- Catalyzed gelcoat (or a premixed kit)
- Pigments for color matching
- Fillers (for structural repairs)
- Acetone (for cleaning)
- PVA or wax paper
Finishing Products
- Polishing compound
- Marine wax
Preparation and safety
Make sure you wear gloves, eye protection and a proper respirator for your own protection. Wash the area with soap and water, rinse, wipe with acetone, and mask a safety margin about 1/16 in. around the perimeter.
Taper and key the edges
For deep dings, use a burr bit to turn the hard fracture line into a smooth U-groove. The taper gives the new gelcoat something to cling to and removes loose flakes. Finish with 150 to 240-grit paper so the surface is dull, not glossy.
Color match and mix
Gelcoat lightens as it cures. Work in small batches by adding pigment drop by drop. Stir in the MEKP catalyst at 1 – 2 % by weight; that gives roughly 10–15 minutes of working time at 70 °F (21 °C). For holes deeper than 3 mm, thicken the mix with silica or micro-balloons until it holds a peak.
Fill slightly proud and seal from air
Squeegee the mix into the cavity, tamping to drive out bubbles, and leave it about 1/32 in. (0.8 mm) proud. Masking tape crowned with PVA, wax paper or clear packaging tape keeps air away so the surface cures tack-free; gelcoat that is exposed to oxygen stays sticky.
Full cure
Let the repair stand overnight, or at least eight hours in warm weather. Do not rush this; catalyzed resin keeps hardening long after it loses tack. If you can dent it with a thumbnail, wait longer.
Fair, finish, and polish
Peel off the film and work down through the grits: 400-wet, 600-wet, 1000-wet. Keep the paper and the surface soaked to float away dust. A lambswool pad with a medium rubbing compound erases the sanding haze; follow with a fine finishing polish and seal the whole area with a UV-stabilized marine wax.
Below waterline and structural considerations
The cosmetic skin of gelcoat is not meant to bridge deep voids. Repairs thicker than about 0.8 mm should be staged: first rebuild the laminate with epoxy or polyester filler, let that cure, then cap it with color matched gelcoat.
Epoxy bonds better and is more waterproof than polyester, making it the material of choice under the waterline as long as you sand it dull before applying the polyester gelcoat on top.
Tools that make the job easier
| Task | Handy Tool | Why It Helps |
| Opening star cracks | 1/8-in. carbide burr in a Dremel | Cuts a controllable V-groove |
| Fairing proud filler | Flexible 3-in. plastic spreader | Follows the curve of the hull |
| Longboard sanding | 12-in. soft board with 120-grit | Prevents wavy spots |
| Final polish | Variable-speed buffer, foam pad | Faster, cooler finish |
Gelcoat maintenance that keeps repairs invisible
Fresh gelcoat is raw and porous until it is sealed. Apply a non-abrasive marine wax every six months.
Between waxings, rinse saltwater off after every trip and use a neutral-pH boat soap. The pay-off? Properly cared for gelcoat can add up to a decade of life to a fiberglass hull.
Frequently asked questions
Does the bottom paint have to come off first?
Only within the area you’ll be sanding; the rest can stay, but rough up the paint edge so the repair feather-blends.
Why not just use automotive clear coat?
Automotive urethanes are thin and too flexible; they crack on a hull that flexes under load. Marine gelcoat matches the chemistry of the factory skin and can be sanded flush.
My boat is dark blue – will the repair always blend?
Color match is hardest with dark shades. Mix several micro-batches, cure them on scrap plastic, and hold them against the hull in daylight before committing.
The bottom line for every boat owner
Minor gelcoat nicks are part of boating life; tackle them early and they stay small. Let them fester and you risk laminate saturation, blister fields and haul-out bills.
Spend an afternoon with sandpaper and a small pot of gelcoat and you will protect your investment and your peace of mind out on the water.
Sea Tow Team
Sea Tow has been the premier leader in on-water boating assistance since 1983. We want to share news, press, tips and all things boating.