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How to Find and Repair a Soft Spot in a Fiberglass Deck

There's a spongy spot in my deck — how do I tell if the core is rotten and how do I fix it?

A spongy deck almost always means water got past a cracked gelcoat or an unsealed deck fitting, soaked the wood core (usually balsa or plywood) sandwiched between the two fiberglass skins, and rotted it — so the skins are no longer bonded to a solid middle and flex underfoot. The fiberglass itself is rarely "broken"; the core has turned to mush and lost its stiffness. A proper repair removes the rot, dries the cavity, bonds in fresh marine core with thickened epoxy, and re-glasses — not just filler smeared on top. Catch it early: a small soft spot is a weekend job, but ignored it spreads and weakens any hardware bolted through that deck (stanchions, cleats, rails).

ℹ️ Reference only: For general reference only. This guide does not guarantee any result — every home is different. Verify against your local building codes and a licensed professional before acting, especially for electrical, gas, plumbing, structural, or roof work.

Common causes

• Water intrusion through unsealed or poorly bedded deck hardware (stanchions, cleats, screws, fittings) — bolt holes wick water straight into the core (most common)
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• Cracked, crazed, or unrepaired gelcoat/stress cracks letting rain and spray reach the core over years (common)
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• Delamination from impact, freeze-thaw cycles, or original-build voids where the skin separated from the core (common)
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• Failed sealant at hatch, port, or deck-to-hull joints draining water into the laminate (less common)
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• Manufacturing defect — dry/under-wetted core or skipped bedding at the factory (rare)
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How to fix it

1. Confirm it's soft, not just flexy: tap-test the area with a small plastic-tipped or phenolic hammer (or a coin). Solid laminate rings sharp and bright; wet/delaminated core thuds dull and flat. Walk the deck and mark the dull-sounding boundary with tape — the rot zone is usually larger than the spot you first felt.
2. Verify with a moisture meter if you can borrow one (a marine/Tramex-type meter). High readings confirm wet core. A surveyor will do this as part of any survey if you want a second opinion before cutting.
3. Decide top-skin vs bottom-skin access. For most deck repairs you cut the TOP skin (gelcoat side) because the underside is often a finished headliner. Plan to cut back at least 1–2 inches into sound, dry, well-bonded core all around the dull zone.
4. Before any cutting, know what's UNDER that deck. Wiring runs, fuel fill and vent hoses, fuel tanks, and plumbing are often routed or mounted directly beneath deck areas — especially side decks and anything near a fuel fill. Cutting or drilling blind can pierce a live wire or a fuel/vent line, which is a shock or fire/explosion risk. Peek from below, trace runs, and disconnect shore power and the battery switch before you start.
5. Cut and remove the top skin over the rot. Set a circular saw or oscillating multi-tool to the skin thickness only (about 1/8–1/4 in) so you don't cut the bottom skin or anything behind it. Lift the skin off in one piece if possible — you can re-bond it later, or make a new skin from glass.
6. Dig out ALL wet and punky core. It must come out down to bare, dry bottom skin. Scrape, chisel, and wire-brush; wet balsa/plywood will be dark, soft, and crumbly. Don't leave any black or spongy material — it will keep rotting.
7. Dry the cavity completely. This is the step people rush. Open it up, use a heat lamp/fan/dehumidifier, and give it days, not hours. Epoxy will not bond to wet glass. Re-check with the moisture meter before going further.
8. Cut new core to fit — marine-grade end-grain balsa or, better for a high-load/through-bolted area, a closed-cell foam (Divinycell/Airex) or marine plywood. Closed-cell foam won't rot if water ever returns, which is why many pros prefer it around hardware. For the immediate footprint of a through-bolted fitting, solid glass or a high-density filler is even better than balsa.
9. Bed the new core in thickened marine epoxy (epoxy + colloidal silica/microfibers to a peanut-butter consistency). Butter the bottom skin, set the core, weight it down so it fully wets out with no voids, and let it cure. Vacuum bagging gives the best bond but clamps/weights work for small jobs.
10. Re-laminate the top skin: grind a tapered scarf bevel around the cut edge (roughly 12:1), then lay up alternating layers of fiberglass cloth and biaxial tape in epoxy, slightly overlapping onto the sound deck, building back to original thickness or a touch proud.
11. Fair, prime, and finish: sand smooth, fill weave with epoxy fairing compound, then refinish. Note: gelcoat is polyester and bonds POORLY to cured epoxy — if you use gelcoat, let the epoxy fully cure, wash off the amine blush with water, and sand thoroughly first, or you'll get peeling. A two-part marine topside paint (e.g. Awlgrip/Interlux) is generally more reliable over an epoxy repair. Add nonskid back to match — an Awlgrip nonskid additive or a KiwiGrip-type coating.
12. Re-bed ALL hardware in the area correctly so this never repeats: oversize each bolt hole, fill with thickened epoxy, re-drill, and bed the fitting in a marine sealant. Use butyl tape or polyurethane (3M 4200 for removable, 5200 for permanent) — never silicone on a structural deck fitting.
13. If the soft area is near chainplates, a mast step, a stanchion base under lifeline load, or anything structural/safety-related, stop and get a surveyor or yard involved before trusting it — a failed stanchion or chainplate base is a fall-overboard or dismasting risk.

DIY or call a pro?

Very DIY-friendly if the spot is small (a square foot or two) and away from structural hardware — it's mostly patience (drying) and basic glass/epoxy work, not specialized skill. Step up to a pro/boatyard when the rot is large, spans a deck-to-hull joint, sits under a finished headliner you can't access, or involves chainplates, mast step, or stanchion/cleat bases that carry safety loads. Also call a pro if you find wiring, fuel/vent lines, or a tank routed right under your cut area, or if the rot is far more widespread than expected once you open it up — extensive core replacement on a coring schedule is a real project.

Tools & parts

• Plastic/phenolic-tipped tap hammer or a coin for sounding
• Moisture meter (marine, e.g. Tramex) — borrow or rent
• Oscillating multi-tool and/or circular saw with depth stop
• Chisels, scrapers, wire brush for core removal
• Heat lamp, fan, and/or dehumidifier for drying
• Marine epoxy resin + hardener (e.g. West System, TotalBoat)
• Colloidal silica / microfibers thickener (for fairing and bonding paste)
• Fiberglass cloth and biaxial tape
• Marine core: end-grain balsa, closed-cell PVC foam (Divinycell/Airex), or marine plywood
• Random-orbital sander + 60–220 grit
• Gelcoat or marine topside paint (Awlgrip/Interlux) + nonskid additive
• Marine bedding sealant: butyl tape or polyurethane (3M 4200/5200) — not silicone
• Respirator, nitrile gloves, eye protection

Based on: BoatUS / BoatUS Foundation; American Boat & Yacht Council (ABYC); West System / Gougeon Brothers epoxy technical guidance; National Marine Manufacturers Association (NMMA); U.S. Coast Guard / USCG Auxiliary boating safety guidance; Society of Accredited Marine Surveyors (SAMS) / marine surveyor practice

General marine-maintenance guidance, not a substitute for a qualified marine technician or surveyor. Boats and conditions vary; for fuel, electrical, fire, or structural issues — or anything safety-critical — consult a professional. Always follow your engine and equipment manuals.