How do you treat boat osmosis and repair hull blisters?

Treating boat osmosis requires removing the blistered gelcoat, washing out the acidic fluid, drying the hull completely, and applying a multi-layer epoxy barrier coat. This labour-intensive job takes several months ashore to allow the underlying laminate to dry before you can safely seal it.

Removing the Gelcoat

Widespread blistering requires removing the gelcoat beneath the waterline entirely. A marine yard will do this using a specialised gelcoat peeler. This costs £40 to £60 per foot of boat length. Peeling removes a precise 1mm to 2mm layer. This takes off the gelcoat and exposes the first layer of chopped strand mat where the osmosis originates. Slurry blasting is another option. It leaves a rougher surface that requires more fairing later.

For localised treatment of a few dozen blisters, you can tackle the job yourself. Use a small angle grinder fitted with a 40-grit flap disc. Grind out each blister into a shallow saucer-like depression. Wear a well-fitted half-mask respirator and a Tyvek suit when grinding fibreglass to protect your lungs and skin from toxic dust. Grind away all the white delaminated fibres until you hit solid laminate.

Washing Out the Acids

Breaking an osmotic blister releases a sharp vinegar-like smell. This comes from acetic and phthalic acid. These acids are a byproduct of water reacting with the un-cured binders in the polyester resin. These chemicals are hygroscopic. They will actively draw more moisture into the hull if left behind.

You cannot let the hull dry with the chemicals still inside. You must wash the exposed laminate.

• Use a pressure washer rated at a minimum of 3000 psi.
• Wash the hull immediately after peeling or grinding.
• Repeat the pressure washing once a week for at least four weeks.
• Hot water washing or steam cleaning accelerates the removal of the glycol fluids.

The Drying Phase

This is the most time-consuming part of the job. The hull must dry until the moisture content drops below 15% on a relative scale. Measure this with a Tramex Skipper Plus or Sovereign moisture meter. In a typical British winter, leaving the boat outdoors to dry takes three to six months.

You can speed this up by tenting the hull with plastic sheeting. Add a desiccant dehumidifier and ceramic heaters. Ensure there is adequate airflow to prevent creating a tropical greenhouse under the boat. Do not apply any filler or epoxy until an independent surveyor or a yard manager confirms the hull is dry. Trapping moisture under a new barrier coat guarantees the blisters will return within two seasons.