Blisters on the hull: osmosis or not?

The boat comes out of the water, the pressure washer goes over it, and the underwater hull is covered in small bubbles. The word “osmosis” comes to mind at once, with all its worries. Yet not all blisters are osmosis, and not all osmosis is equal: many blisters only affect the paint, and mild osmosis can be monitored for years. This guide helps you tell the difference and know when to bring in a professional. It concerns polyester hulls (glass-reinforced plastic); for other materials, the causes are different.

1. What you notice

• Small blisters in the antifouling, which squash or burst easily.
• Hard blisters, from the size of a lentil to that of a coin, which seem to come from under the paint.
• A burst blister that lets out a liquid, sometimes under pressure, with a sour, vinegar-like smell.
• Blisters grouped in certain areas (below the waterline, near the keel, on the rudder) or spread everywhere.
• A remark from a yard or a surveyor about the hull’s high moisture content.

2. Safety first

Blisters almost never put the boat in danger in the short term. Two exceptions: large blisters with an area that sounds hollow or gives under your finger, especially around the keel, the rudder or an old repair, can signal delamination (layers of the laminate coming apart) — have them looked at before sailing. And sanding antifouling produces dust loaded with biocides: a suitable mask, glasses, overalls, and compliance with the boatyard’s rules on collecting waste.

3. The probable causes, from most to least common

• Paint or antifouling blisters — the antifouling or paint layer comes away from the layer below: paint applied to a damp, greasy or badly sanded surface, incompatible products, too many coats built up over the years. Under the blister, you find an old layer of sound, dry paint. This is the most common case, and it is harmless to the hull.
• Blisters between the primer and the gelcoat — a primer (protective undercoat, often epoxy) badly applied or applied to a damp gelcoat lifts. The gelcoat underneath is intact; it is an application fault, not osmosis.
• Gelcoat osmosis — water passes very slowly through the gelcoat (the smooth outer layer) and reacts with components that have remained soluble in the laminate; small pockets of liquid under pressure form, with the characteristic sour smell. The blisters are hard, often numerous and of regular size.
• Deeper osmosis in the laminate — when the process has progressed, the blisters are in the fibres themselves, larger, sometimes with areas coming apart. This is rarer and more serious.
• Other defects — dry air bubbles trapped during manufacture, gelcoat coming away in places, an old impact or a poor repair. The blisters are then dry and localised.

Severity varies a great deal. A few isolated blisters in the gelcoat do not carry the same weight as a hull riddled with them or a damaged laminate. A full osmosis treatment is a major job: the gelcoat is removed, the hull is left to dry for a long time (weeks, often months), then a protective barrier is rebuilt, usually epoxy-based. Depending on the case, a professional will advise that treatment, a local repair of the blisters, or simply monitoring from year to year.

4. What you can check yourself, safely

• Photograph the underwater hull as soon as it comes out of the water, area by area, with a reference (tape measure, known object) for the size of the blisters.
• Note where they are (bow, stern, below the waterline, near the keel, on the rudder), how many, and their size; compare with photos from previous haul-outs if you have them.
• Press gently: is the blister soft (paint lifting) or hard? Is it dry, or does it smell of vinegar if it is already open?
• Do not pierce at random. If you want to know which layer a blister comes from, open only one, carefully chosen, with glasses on, and look at what is underneath: old paint, primer, gelcoat or fibres. It is still best to let the surveyor do it.
• Gather the hull’s history: haul-out dates, number of antifouling coats, last strip, whether an epoxy primer has been applied, any previous osmosis treatment and its invoices.
• If a moisture meter is to be used, wait until the boat has been ashore for a while: a reading taken just after coming out of the water is always high and means almost nothing. The meter gives a relative value, to be compared between areas (for example with an area above the waterline); how to read it depends on the meter, the paint and the temperature. Leave the measurement and its interpretation to a professional.

5. When to stop and call a professional

Bring in a professional if the blisters are hard and numerous, if sour liquid comes out of them, if areas sound hollow, if the blisters spread from one haul-out to the next, or before deciding on a treatment. When buying, the question is settled by a survey ashore, with moisture readings and a few blisters opened by the surveyor. A haul-out yard will be able to tell whether it is the antifouling that is blistering and when to strip it (see haul-out and antifouling).

6. Stopping it from coming back

A hull that spends part of the year ashore, well ventilated, ages better. When the antifouling has built up too many coats, stripping down to the gelcoat is the opportunity to check it and to apply a protective epoxy primer, on a thoroughly dry hull and following the paint manufacturer’s data sheet. Keep the bilges dry, and look at the underwater hull at every haul-out with the same method: photos, mapping, and moisture readings taken in the same conditions. At sale time, visible blisters will be noticed by the buyer and by their surveyor: a known, documented situation is better than a discovery that fuels a negotiation or a dispute (see hidden defects). A completed treatment, with its invoices, its moisture readings before and after and the yard’s guarantee, is on the contrary a selling point.

In the Hullteam log, record every haul-out, the blisters you saw with their photos, the moisture readings (date, meter, conditions) and any treatment: that is what will make the boat’s value at resale (see which proofs count).