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Deck

Repairing a damp deck core (balsa/foam)

Diagnosing and repairing a damp core in a sandwich laminate (balsa/foam): drying and rebuilding.

The sailing catamaran Madras

The decks and coachroofs of most yachts are sandwich laminates: two laminate skins with a light core of plywood, balsa or foam. The construction is stiff and light, but once water gets into the core through an unsealed fitting or a crack, balsa rots and plywood delaminates. The result is a soft, 'spongy' deck and a loss of stiffness. Repairing a wet core is one of the harder laminating jobs, demanding patience and precision.

Author: Maciej Baniewicz · Boat & Yard editorial team

Methodology: The material was prepared from the sources given and from practical service rules; the parameters have to be confirmed in the manual for the specific manufacturer or model.

Diagnosis starts with tapping the deck — a sound core gives a sharp, high note, while a wet or detached one sounds dull and 'plastic'. Map the suspect areas and confirm them with a moisture meter (a laminate detector) or test holes, judging the colour and smell of the shavings. The source of the water is often badly sealed pulpits, cleats, stanchions, hatches or the mast step. Without finding and curing the leak, every repair will be temporary.

The repair strategy depends on access and scale. Most often the top laminate skin over the damaged area is cut out (from the deck side), the wet, ruined core removed back to sound material and the inner skin exposed. Smaller pockets can be repaired from below or by injecting resin, but with extensive balsa rot that is a half-measure. The edges of the cut are chamfered (the laminate stepped), so that the new repair bonds well to the old structure.

The exposed area must be completely dry before rebuilding — often the longest stage, taking weeks, helped along by ventilation, heating and sometimes warming the laminate. Moisture trapped in the core will destroy the new repair and prevent the resin from bonding. The new core (ideally a water-resistant structural foam such as PVC, or balsa in less exposed places) is bonded in with epoxy and filler, making sure it is fully bedded with no voids. Voids under the core are future detachments.

Once the core is bonded in, the outer laminate is rebuilt with glass mats and cloths wetted out with resin (epoxy, for its adhesion and water resistance), layer by layer, matching the weights of the original. The surface is sanded, filled and returned to its original shape and texture, including the non-slip areas. Finally a paint system or gelcoat goes on. Precision at the laminating stage decides the strength — a repaired deck has to carry the loads just as the original did.

Preventing a recurrence is crucial. Seal every hole for a fitting with the correct technique: drill it oversize, fill it with epoxy and filler, drill again in the cured resin and only then fit the hardware on flexible sealant. That way the core is isolated from water even when the fitting's seal eventually leaks. It is a standard that eliminates most future water ingress. Regular inspection of the seals extends the life of the deck.

Safety: working with resins and glass fibre and sanding laminate means exposure to dust and fumes that irritate the airways and the skin — a mask with an organic vapour filter, a coverall, gloves and goggles are compulsory. Epoxy resins are sensitising, so avoid skin contact and work with ventilation. Dust from sanding laminate is harmful — use extraction and a well-sealed dust mask. Given the scale and the consequences, extensive structural repairs are worth discussing with, or leaving to, a professional workshop.

Material prices — products and options

Indicative prices in EUR (incl. VAT), as of July 2026. Products widely available in Europe; prices depend on the shop, the pack size and consumption. The links lead to example shops that ship across the EU. Dollar amounts are converted at 1 EUR = 1.1622 USD (ECB, 7 September 2026) and rounded — treat them as a rough guide, not a price in a US shop. Imperial values in brackets are conversions of the metric figure.

Core: structural PVC foam or balsa
Epoxy resin with hardener
Fillers (microballoons, cabosil) for bonding and filling
Glass mats and cloths of various weights
Gelcoat or a topcoat paint system
Injection resin / core consolidation
Laminate moisture meter
Tools: sander, jigsaw, de-airing rollers
Safety gear: mask with an organic filter, coverall, gloves

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