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Complete Guide For Anti-Osmosis Treatment on a Boat

Osmosis is a natural phenomenon that results in the penetration of water into the inner layers of the hull. More critical in freshwater than in saltwater, it mainly affects “plastic” boats made of polyester resin and fiberglass. Vessels made of orthophthalic resins (produced before 1990) are more likely to be affected by this problem. Although this phenomenon does not prevent navigation, it is strongly recommended to carry out an anti-osmosis treatment to avoid exposure and degradation of the polyester. 

The appearance of osmosis causes some anxiety in the owner of the boat. (as an individual) I have never seen a boat sunk because of osmosis. This phenomenon is prolonged and before we get there, we have plenty of time to deal with the problem. Thousands of boats sail with blisters on the hull without any problem. So do not panic, I show you how to do an anti-osmosis treatment yourself. Our how-to troubleshoot an outboard motor alignment will help you in proper alignment.

Anti-osmosis treatment on a boat: The complete cure

Osmosis is mainly due to a difference in concentration between two liquid media. This is seawater (or freshwater) and an acid solution resulting from the hydrolysis of the resin component of the hull. The two media are separated by the gel coat, which loses its properties and allows fluids to pass from both sides until the osmotic equilibrium (concentration equilibrium) is reached.

And yet, it is a natural phenomenon caused by the aging of the polyester fibers and the degradation of the Gelcoat layer that becomes partially permeable. Moreover, it is not unusual to see the appearance of osmosis on boats from 10 to 15 years.

Before proceeding with the restoration of the hull, it is essential to evaluate the damage. Indeed, the quality and durability of an anti-osmosis treatment usually depend on a relevant assessment of the damage to the shell. So, how to detect osmosis on a boat?

First of all, it must be known that this phenomenon is manifested by the appearance of blisters containing acetic acid on the surface of the shell. Once pierced, a blister emits a strong smell of vinegar and releases resin solvents. Over time, the gel coat at the level of the blisters explodes revealing cracks in the polyester.

The appearance of blisters is a symptom that reveals the presence of osmosis on your boat. That said, to accurately assess the severity of this problem, it is necessary to measure the humidity level at the hull using a hygrometer.

High humidity usually indicates high porosity in the first layers of the laminated polyester. To refine the evaluation of the damage, you can plan (by sandblasting or peeling) some blisters to see more precisely the state of the polyester. In general, the anti-osmosis curative treatment  is done in 3 steps:

1- Exposure of the hull

For starters, it is imperative to take the boat out of the water at the shipyard. Thoroughly wash the hull with a pressure washer with a rotating nozzle.

Then there are 2 cases, or there are small isolated blisters, at that moment it is enough to treat locally. Either there are blisters on the whole hull and in this case, it is necessary to eliminate the entire layer of the gel coat with the help of an electric plane (rented). You can also sandblast at high pressure. This operation would give almost the same results but know that it is relatively more expensive.

The idea is to bare the hull to access the layers of fibers to let the water and acetic acid trapped inside the hull escape.

It is, therefore, a crucial step that largely determines the quality of the treatment. Be careful not to remove the polyester as this may weaken the structure of the hull. The use of the plane is very delicate, pay careful attention to the setting of it. Doing an anti-osmosis treatment is not very complicated, but it takes a lot of concentration at this stage. Do not forget to protect yourself with a complete suit, gloves, glasses and especially a mask cartridge (more effective than simple masks that let a lot of dust).

Then wash the boat with a basic solution (pH greater than 7) and high-pressure jet at about 300 bar to further neutralize the acetic acid and remove all residues of the gel coat. If the osmosis touches only a few parts of the hull, then you can follow the same procedure to perform a local treatment of the blistering to reduce the effort and the costs.

2- Drying the hull

After removal of the gelcoat layer, allow the hull to dry in a ventilated place. The drying time of the hull varies depending on several factors. In particular the ambient temperature, the climatic conditions, the humidity … etc. It will take at least 2 to 3 months and check the moisture on the hull at least once a week to ensure the excellent quality of drying.

For advanced osmosis conditions, the hull should generally be allowed to dry for 6 to 12 months. It is advisable to use a powerful dehumidifier or a hot-air blower to accelerate drying. 

3- Anti-osmosis treatment of the shell: the finish

After drying, all the stratification defects in the shell must be corrected. Carefully locate and clear any cavities that appear on the outer layers of the laminate. Then use a hygrometer to measure the moisture content.

The next step is to chew the hull to maintain its original shape. This step is quite long and requires a good look to apply the epoxy putty in the right places for a homogeneous hull. The entire hull should then be sanded with an orbital sander at 220 grit.

Come to the step that aims to isolate the hull of the water, for this; it is necessary to apply the epoxy resin in 2 or 3 layers (without solvent) on the whole hull. The recommended product is an epoxy primer as International Interprotect. The successive layers must be applied by spun for better adhesion. This work is undoubtedly tricky, but it is within reach of an experienced handyman. 

It is always advisable to carry out this operation in a shed to control all the humidity and temperature conditions. Once the epoxy primer treatment has been completed, it is a very impervious protective barrier against moisture. Finally, sand the last layer of epoxy, VERY lightly and apply to antifoul adapted to your boat. 

Preventive treatment of osmosis on a boat

To fight osmosis effectively, a precautionary approach is needed. Regular inspection of the hull that can detect a possible tendency of the polyester to absorb water.

If you notice microcracks, pinholes, or star – spalling above the waterline, you must act. Indeed, these defects are often difficult to detect below the waterline because they are masked by antifouling. They usually indicate cracks in the gel coat and mark the beginning of the osmosis process.

To minimize the damage, it is necessary to carry out a preventive osmosis treatment. To do this, dry the shell, strip the existing antifouling and apply 2 to 3 layers of epoxy primer. Then sand the outer layer of the introduction and apply two coats of antifouling. 

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Osmosis treatment: what to remember

Osmosis is a natural phenomenon that is manifested by the appearance of blisters in the gel coat. In the case of advanced osmosis, it is necessary to eliminate the entire gel coat. Then let the hull dry.

After drying, 2 to 3 coats of epoxy primer should be applied and antifouled. When blistering does not affect the whole hull, it is possible to make a local treatment to minimize the effort and costs. I will explain in an upcoming article how to do a residential treatment of osmosis.

In any case, the state of the hull should be regularly checked to anticipate the appearance of osmosis. You should also store the boat ashore when not in use if you have the option.

The osmosis can be favored by leaks of water on board. It is therefore recommended to check all the water circuits onboard the boat regularly. This will detect any leaks and avoid having water in the funds. You may like the 5 Things that Keep Us from Going to Sea Fast

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