Osmosis in a laminate hull — diagnosis and repair
Diagnosing and fully repairing osmosis in a polyester laminate hull — drying, sanding, barrier coat.
Osmosis is the process in which water penetrates the gelcoat into the laminate and reacts with the residues of uncured components of the polyester resin and with glycols. Acid solutions form and create blisters under the gelcoat, accompanied by a characteristic vinegary, acid smell when they are pricked. The problem mainly affects hulls in orthophthalic resins from the 1970s to 1990s, though newer boats in vinylester resin are far more resistant. Osmosis rarely threatens a hull's strength overnight, but ignored it leads to delamination and loss of stiffness.
Diagnosis begins once the yacht is out of the water, ideally in early spring while the laminate is still saturated. You feel and look for blisters over the whole underwater part of the hull, and measure the moisture with a meter for laminates, for example from Tramex. The values have to be compared between the underwater part and a reference area above the waterline — a difference of a dozen or so points indicates water saturation. A single pricked blister releasing sticky, acid fluid confirms a diagnosis of osmosis rather than ordinary mechanical damage.
The first stage of the repair is removing the gelcoat from the whole underwater surface. Most often a special gelcoat plane (gelplane) or a sander with coarse 40–60 paper is used, taking the layer off until clean, blister-free laminate is exposed. The work is dirty and dusty — a P3 filter mask, goggles and a coverall are absolutely required, because glass and resin dust is an irritant. After sanding, the hull has to be washed thoroughly with fresh water under pressure, to flush the acid glycols out of the pores of the laminate.
The key and longest stage is drying the hull. The laminate must dry under a roof or a tent, out of the rain, often for 4–12 weeks, and a dehumidifier and gentle heating speed the process up. The moisture is checked with the meter every few days — the target is values close to those above the waterline (usually below about 15 units on the meter's relative scale). Washing periodically with warm water while drying helps draw the remaining glycols to the surface; if efflorescence appears after drying and re-wetting, the drying has to continue.
Once dry, the hollows and unevenness are filled with epoxy filler (based on the WEST System with fillers, for example), and deeper pits are laminated with glass cloth and epoxy resin. Then the barrier coat goes on — several coats (usually 4–6) of epoxy barrier primer, such as International Gelshield 200 or Hempel, to reach the recommended dry film thickness of about 300–500 microns. The coats are applied wet on wet within the windows the maker specifies, watching the temperature and humidity. It is epoxy, not polyester, that forms an effective barrier to water.
Finally, once the barrier coat has been sanded and degreased, a primer for the antifouling goes on, followed by the antifouling itself. It is worth keeping a work log with dates, temperatures and coating thicknesses — that helps with any warranty claim and with future maintenance. With the repair done properly and good drying, the result lasts many seasons. Remember that rushing the drying stage is the commonest reason osmosis comes back — this is not the place to save time.
The whole operation needs stable conditions: temperatures above 10–15°C (50–59°F) and low relative humidity when applying resins and coatings. On large areas, consider dividing the hull into sections and working in stages, to stay within the resins' working windows. If the hull is heavily saturated or has extensive delamination, it is worth discussing the scope of the work with an experienced laminator. A properly executed osmosis repair is an investment that genuinely raises the yacht's value and safety.
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.
Removing gelcoat and sanding
- Gelcoat plane (gelplane)
hand plane for taking off the gelcoat layer in osmosis work€40–120 (≈$46–139) - Random orbital sander + 40–80 grit
for sanding back the laminate and barrier coats€60–200 (≈$70–232)
Measuring the laminate's moisture
- Tramex — Skipper / Marine Moisture Meter
the reference moisture meter for GRP, monitoring the drying€200–380 (≈$232–442) - Budget material moisture meter
a cheaper indicative instrument for relative comparisons€30–80 (≈$35–93)
Epoxy resin and fillers
- WEST System — 105 + 205 (1.2 kg (2.6 lb))
resin + fast hardener for repairs and laminating€70–80 (≈$81–93) - WEST System — 105 + 205 (6 kg (13.2 lb))
pack for larger hull areas€250–320 (≈$291–372) - International — Epiglass epoxy resin
alternative resin for laminating and barrier coats€100–130 (≈$116–151) - Epoxy Fiber Filler — epoxy filler
filler for deep hollows and levelling€15–40 (≈$17–46)
Glass cloth and mat
- SEATEC — glass cloth 200 g (7 oz)/m²
for local repairs with epoxy resin€10–20 (≈$12–23) / m² - Chopped strand mat 450 g (16 oz)/m²
mat for polyester laminates (not for epoxy)€5–12 (≈$6–14) / m² - SEATEC — epoxy + cloth repair kit
set for small laminate repairs, 250 g (9 oz)€20–25 (≈$23–29)
Anti-osmosis barrier primer
- International — Interprotect
two-pack epoxy primer, moisture barrier (base)€100–140 (≈$116–163) - International — Gelshield 200
epoxy barrier coating below the waterline, 750 ml (25 fl oz) / 2.5 l (0.7 US gal)€45–130 (≈$52–151) - Hempel — Light Primer (2K epoxy)
high-build anti-osmosis primer, also for spot repair€25–90 (≈$29–105)
Undercoat primer and the final antifouling
- Hempel — Underwater Primer (1K)
fast-drying primer under antifouling€20–40 (≈$23–46) - Hempel — Classic Antifouling
self-polishing antifouling coating, 0.75 l (0.2 US gal)€24–55 (≈$28–64) - International — Micron / Cruiser Antifouling
popular antifouling coatings, various versions€35–90 (≈$41–105)
Personal protection
- Mask / half mask with an A2P3 filter
protection from resin fumes and laminate dust€25–70 (≈$29–81) - Nitrile gloves + goggles + coverall
disposable protection when working with epoxy€10–30 (≈$12–35)
Price sources: svb24.com, compass24.com, the makers' sites (West System, International, Hempel, Tramex). Prices depend on the pack size and the shop.
Common questions
How do you tell osmosis from ordinary damage?
By what is inside the blister. A pierced osmotic blister releases a sticky fluid with a sharp, vinegary smell — that confirms the diagnosis. A laminate moisture meter also reads clearly higher below the waterline than on the reference topsides.
When is the best time to check for osmosis?
Right after the boat comes out of the water, ideally in early spring while the laminate is still saturated. After a few weeks ashore the hull dries out and moisture readings no longer show the real state.
How long does drying the hull take?
Usually four to twelve weeks under a shelter or tent, out of the rain; a dehumidifier and gentle heat shorten it. The criterion is not the calendar but the meter: moisture has to approach the readings from the topsides. If efflorescence appears after drying and re-wetting, drying continues.
Why does the barrier coat have to be epoxy?
Because it is epoxy, not polyester, that forms an effective barrier against water. You normally apply four to six coats of epoxy barrier primer, wet on wet within the maker's time windows, to a recommended dry film thickness of around 300–500 microns.
Is every GRP hull at risk?
Not equally. The problem mainly affects hulls in orthophthalic resins from the 1970s to the 1990s; newer boats in vinylester resin are far more resistant. Osmosis rarely threatens strength overnight, but ignored it leads to delamination and loss of stiffness.