Gelcoat Crack Repair: From Spider Cracks to Gouges
First, diagnose the crack
Gelcoat is a cosmetic skin over structural laminate. The repair only lasts if you know why it cracked:
- Spider/crazing webs radiating from a point: impact from the other side or a stressed fastener.
- Parallel cracks along a chine, corner, or bulkhead line: the laminate is flexing — a structural issue wearing a cosmetic costume. Fix the flex or the cracks return.
- Random crazing over large areas on older boats: aged, over-thick gelcoat; manage expectations.
- Cracks at deck hardware: loose or unbedded fasteners; rebed the hardware as part of the job.
Tap-test and moisture-check around any crack field. If the laminate sounds dull or the core is wet, this is not a gelcoat job — see the transom/core assessment article.
Cosmetic repair procedure
- Open the crack: chase it with a rotary V-bit or scraper so the repair has something to hold — filling an unopened hairline guarantees it telegraphs back.
- Clean with acetone; tape off generously.
- Fill with gelcoat paste tinted to match (color matching is the craft — test on tape before committing; white is never just white).
- Gelcoat won't fully cure exposed to air: cover with PVA or film per the product instructions, or use a waxed formulation.
- Cure, then wet-sand through the grits and buff to blend. Feather wide — a small repair with a wide blend disappears; a proud patch with a hard edge is forever visible.
Below the waterline / gouges into laminate
Gouges through gelcoat into glass need laminate repair first (epoxy or polyester per the situation) and barrier considerations below the waterline. Note that gelcoat over epoxy has adhesion caveats — follow the resin manufacturer's guidance.
Common mistakes
- Filling stress cracks without fixing the flexing structure
- Skipping the air-inhibition step and wondering why it stays tacky
- Color-matching under shop lights instead of daylight
- Promising "invisible" on a 20-year-old sun-faded hull — promise "very good" instead
When to walk away
Crack fields with wet core, cracks reappearing after a prior repair, or structural grid cracks in the bilge are surveyor/glass-shop territory. Cosmetic mobile repair has limits — knowing them is professionalism.
Safety: acetone, resins, and sanding dust want gloves, eye protection, and a respirator; ventilate and control dust around the customer's dock neighbors.