Osmosis in fiberglass hulls: how to detect it early and avoid bigger issues

Osmosis is one of those problems no boat owner wants to encounter. It appears gradually, often goes unnoticed at first and, if left untreated, ends up compromising the performance, safety and value of your vessel. The good news is that with early detection and the right treatment, it can be solved for good.

In this article we explain exactly what osmosis is, how to recognise the first signs, what checks we perform in the shipyard to confirm it and which professional treatment really works.

What is osmosis in a fiberglass hull

In fiberglass and polyester resin hulls, water can slowly penetrate through the gelcoat. This moisture reacts with components of the resin, forming acidic fluids in small internal cavities. Over time, those cavities turn into visible blisters on the hull.

Although they may look like a cosmetic issue, they are in fact the visible symptom of a chemical process that deteriorates the laminate, adds weight to the boat and increases drag while sailing.

Warning signs every boat owner should check

There are clear indicators worth reviewing during each haul-out:

• Small blisters on the hull, especially below the waterline.

• An acidic or vinegar-like smell when opening a blister.

• Areas of paint that bulge or peel off for no obvious reason.

• Patches that seem to “weep” moisture after sanding.

• Loss of performance: higher fuel consumption to maintain the same speed or a general feeling of a “heavier” boat.

If you notice any of these symptoms, it’s best to schedule an inspection. The sooner action is taken, the simpler the intervention will be.

How we confirm osmosis at the shipyard

A serious evaluation combines observation, mechanical testing and measurements:

• Visual inspection: the hull is cleaned and suspect areas are marked.

• Percussion: using a small hammer to detect if the laminate feels soft or hollow.

• Moisture readings: specific equipment measures different points of the hull and results are compared.

• Sample opening: in advanced cases, a blister is opened to analyse the liquid inside and the condition of the fibre.

With this information we create a map of the hull and determine the best treatment strategy.

Why it’s important not to delay treatment

Osmosis does not stop on its own or “dry out” while sailing. It is a progressive process: water that enters through the gelcoat reacts with the resin, generates acidic fluids in micro-cavities and keeps the cycle active. Over time, blisters increase in number and size, new areas are affected and the problem goes from superficial to structural. The longer it is left, the larger the surface to repair and the more complex the work becomes.

This progression is also noticeable in performance. A hull with blisters and porosity is rougher and drags more water; the boat feels sluggish, needs more throttle to maintain speed and manoeuvres less smoothly. It’s not just aesthetics: that extra drag means higher fuel consumption and worse handling, especially on long passages or in heavy seas.

From a resale point of view, postponing treatment means any serious inspection will highlight it in the report, which complicates the sale or leads to lower offers. Some insurers and surveyors, when faced with untreated widespread osmosis, may even condition coverage or recommend intervention before granting it. Having a documented, treated hull with a recent epoxy barrier, on the other hand, is a strong argument in your favour.

Finally, there’s the matter of laminate integrity. If moisture and chemical reactions are allowed to act for months or years, soft spots, local delamination and loss of laminate thickness may appear. At that stage, patch repairs are no longer enough: the hull must be peeled, thoroughly dried and rebuilt over larger areas, which means more time ashore and greater technical complexity.

In short: acting early means containing the problem, reducing labour hours, cutting down time out of the water and gaining long-term reliability. If you’ve seen blisters or clear signs, the smart move is to schedule an inspection and book a work slot at the end of the season or a few weeks before the next.

Professional treatment step by step

The process requires time, expertise and specialised equipment. This is how we tackle it at the shipyard:

1. Haul-out and stripping: antifouling is removed and the hull is thoroughly cleaned.

2. Gelcoat removal: affected layers are peeled off to expose the laminate.

3. Washing and neutralisation: acidic residues are removed to stop the reaction.

4. Controlled drying: the hull is left to dry using ventilation and dehumidification systems. This is the most crucial phase.

5. Laminate repair: damaged areas are rebuilt with the right fabrics and resins.

6. Epoxy barrier: several coats are applied to ensure impermeability.

7. Antifouling finish: the hull is primed, painted and ready to go back into the water.

Total time depends on the initial moisture level, the climate and the severity of the case. Sometimes a few weeks are enough; in more severe cases, drying may take longer.

How to prevent osmosis in the future

While it’s not always possible to avoid osmosis 100%, good practices reduce the risk significantly:

• Keep an epoxy barrier in good condition.

• Inspect the hull carefully at each haul-out.

• Renew antifouling properly, with correct surface preparation.

• Avoid long stays in freshwater if your boat is usually in saltwater.

• Request moisture readings periodically, especially on second-hand boats.

These habits extend the life of your hull and help avoid unpleasant surprises.

Frequently asked questions

Are all blisters signs of osmosis?
No. Some are caused by paint incompatibility or poor preparation. The difference is that osmosis blisters contain acidic liquid and affect the laminate.

Can osmosis return after treatment?
If drying is done properly and a strong epoxy barrier is applied, recurrence is very unlikely.

Can I keep sailing with osmosis?
It depends on the extent. If there are just a few blisters, you may plan treatment at the end of the season. If damage is widespread, early intervention is the responsible choice.

Is it worth repairing?
Yes. A clear diagnosis helps you decide, but most treated boats recover their performance and market value.

Regain peace of mind on the water

Osmosis doesn’t have to turn into a major problem. Detecting it early and trusting a professional treatment restores your hull’s integrity and lets you sail with peace of mind.

If you’ve noticed blisters or suspicious signs on your boat, we can help you plan the intervention at our shipyard in Barcelona and leave your vessel ready for the next season.