Osmosis In Boats

Treatment of osmosis

Will it cause my boat to sink? Will my boat be unsaleble?

The term "osmosis" often refers to a biological process where a liquid (usually water) passes through a semi permeable membrane. The water molecules pass through the membrane, but not the compounds dissolved in the water and the flow will be from the solution of lowest concentration towards that of higher concentration. In nature osmosis is used by plants to draw moisture and nutrients from the soil.

The word was coined in the early 70s to describe the blistering found on many GRP (glass reinforced plastic) boats. GRP must be a close to perfect boat building material. It is relatively cheap, lightweight but strong, can be moulded into complex shapes and is almost maintenance free. Nevertheless osmosis can cause problems in GRP boats, although these are often not as great as the scaremongering and old wives tales would have you believe. In boats the membrane is a polyester gelcoat or epoxy paint and when the stronger solution draws solvent from the weaker one it becomes diluted and increases in volume and pressure, which leads to the familiar gelcoat blistering. The osmotic process can be reversed by applying greater pressure than the osmotic pressure, or by swapping the two solutions around. But this does not answer all the questions:

  • You may wonder how water can pass through a polyester gelcoat or epoxy coating when these materials are described as impermeable to water. The answer is that all paint coatings have minute gaps or holes in their molecular framework, allowing tiny quantities of moisture to pass through.
  • Even though a brand new GRP hull is, in theory, chemically inert, it will still start to absorb moisture through the gelcoat as soon as it goes in the water. Very little damage will be caused at first, probably for the first two or three seasons. An epoxy resin may have a lifespan of ten or even twenty years if it is applied properly and not damaged. Eventually however, there can be a breakdown of the components within the gelcoat and while the hull may still appear to be in good condition, tiny amounts of moisture below it will be trying to destroy the laminate by breaking it down into its original constituents. These products will contain acids which give osmotic blisters their characteristic “vinegar” odour and will also add to “osmotic pressure”. While drying the boat out may provide a temporary reduction to moisture content, it will do nothing to remove the chemicals in solution which are the real cause of the problem.
  • Eventually the cycle of moisture absorption and laminate breakdown reaches the stage where moisture is absorbed more quickly than it can escape and that is when the gelcoat starts to blister. It usually happens slowly in small areas of the vessel’s bottom but will gradually increase. Localised treatment can be tried but this is only a temporary solution as it is likely that most of the vessel’s hull will be in a similar condition and will soon begin to show the tell tale blistering.
  • Although osmosis does not cause de-lamination, the internal pressure caused by the osmotic process may separate poorly adherent layers from each other causing large swellings in the hull surface. The hull may also feel slightly soft when pressed firmly and may sound “dull” if tapped gently.

The effects on your boat

Will osmosis make it sink?

While there are many horror stories of boats sinking on their moorings, or absorbing so much water they can hardly float, these are, on the whole, exaggerated. Gelcoats have very little mechanical strength but do provide a hardwearing exterior finish, helping to protect the structural laminate beneath it. Most of the absorbed moisture and solutes are confined to the layer directly behind the gelcoat. The quantities of moisture involved are unlikely to have any adverse effect on buoyancy and the majority yachts can be sailed safely with their gelcoats completely removed. If there are symptoms of de-lamination, however, they must be investigated without delay.


Osmotic blisters may vary in size from a pinhead to 10cm in diameter and will almost always be filled with fluid. This should be tested for acidity using pH papers and any reading between pH 0 to pH 6.5 suggests an osmotic condition. If the fluid has a sticky or greasy feel it indicates the presence of glycol, which must be completely removed in order to dry out the laminate and treat it. Sections of the gelcoat should also be removed in order to examine the laminate itself and find out how deeply seated the problem is. Some older boats may have as much as 5mm depth of the hull affected by osmosis. In these cases an expensive re-lamination may be the most sensible option.

Although the well known saying, “a stitch in time saves nine”, is usually true, in the case of treatment of osmosis, early treatment often seems to be less successful than treatment of boats with advanced blistering. This is because the breakdown process takes time and if treatment is carried out too early a reoccurrence of the problem is more likely to occur. Laminates are also easier to treat after a season afloat than after a long period out of the water.

The main thing to remember is that the strength of your hull should not be endangered by letting the condition deteriorate too far.

How Do I Go About It?

The damaged or weak areas of laminate can be removed by abrasive grit blasting or slurry blasting. This process will not disturb the surrounding sound areas of the boat. Blasting will produce an excellent surface with good adhesion properties for paint coatings. Unfortunately blasting is a messy and slow job which has been heavily restricted in many marina complexes. It may also produce an uneven hull profile which may need filling.

Another method is to use gelcoat peelers, which are comparatively quick and clean and produce a smooth hull profile. This method too has its disadvantages. It only removes a pre-set thickness of material, so may not remove all the suspect areas. It also means that the smooth surface produced may make drying and removal of solutes more difficult. Probably the best compromise is to use a gelcoat peeler first, to remove the majority of unwanted gelcoat and laminate and follow up with some moderate grit blasting to prepare the surface, using a pressure washer with a suitable attachment.

Grinding is another method, useful for preparing small areas, but its disadvantage is that it produces large quantities of dangerous dust, so respiratory protection must be worn.

Heat guns should NOT be used as they might result in distortion and de-lamination, especially if the hull is wet. Heating GRP can also generate toxic fumes, especially if old antifouling is present.

After preparing the surface of the hull, the next stage is to dry it out, preferably in natural conditions, making sure that the bilges are also dry and well ventilated. Taking moisture readings may help to confirm that the hull is dry enough to treat.

Solvent free epoxies are the usual recommended initial coating for bare laminates as they avoid the risk of solvent retention and have good impregnation and adhesion properties. They can also be used for subsequent coats and you can apply a new coat as soon as the previous one is tacky. You may however prefer to use a solvent containing gelshield which has a better tolerance to lower temperatures, more flexible over-coating times and good performance where moisture permeability is concerned. If you are applying antifouling as well make sure that you use materials which are compatible. Some antifoulings have poor adhesion to epoxy coatings.


Beware of believing everything you hear down at the harbour or read on the internet, such as:

  • Boats don’t get osmosis.
  • It will go away by itself.
  • It’s just a slight humidity problem.
  • Your boat could sink.
  • You won’t be able to sell it.
  • It only happens in old boats
  • ...etc.

My advice is – don’t panic when you see the first blisters appear. Monitor them, test them, get advice from someone reliable, who you can trust and who knows their boats. When you decide that the time is right to treat the patient, do a thorough job, don't skimp it or go for the cheapest option.

Author – Dee White

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