2.1. Hulls covered with epoxy or other type water shield.
2.2. Hulls that have been relaminated.
2.3. Hulls repaired without relamination.

There are far too many circumstances involved in the "osmosis" process and too many different FRP hull constructions to allow for a complete summary of the subject.

Here only the most common cases found during 25 years of practical "osmosis" repair and research will be described.

2.1. Hulls covered with epoxy or other type water shield.

2.1.1. Small blisters similar to the original ones.

The repair done was limited to shallow removal of the gel coat blisters, filling and applying of a water shield barrier.
The actual "osmosis" process in the laminate has proceeded totally unchanged, and may affect many deeper laminate layers.

2.1.2. Small local blisters + some larger in the bottom part.

The repair might have included blasting or peeling and sufficient drying, but acid has been left both inside the remaining laminate and smeared in spots on the surface.

Only filler and water shield have been applied.  The "osmosis" spreads very fast on the total surface between the laminate and the water shied.

Much of the water shield is separated from the laminate and permits the "osmosis" fluids to pass down to the bottom of the hull instead of forming blisters in the upper parts. Especially in older cases where tar epoxy has been used, the bottom blisters may be big as bags.

Deeper laminates may be affected but mostly only spot wise.

2.1.3. Hard coatings will not blister.

Very often it is not detected until the hull feels flexible or on sailing yachts the keel becomes unstable. If more than 4-5 years have passed since the repair, a new repair might be too expensive compared to the value of the boat.

In "lucky" cases where some areas of the laminate were smeared with acid when the coating was effected, hand sized blisters may give earlier warning.

2.2. Hulls that have been relaminated.

2.2.1. Small blisters only in water shield.

If the water shield on top of the new laminate blisters, either

1. : A solvent type epoxy has been used underneath or
2. : There have been so much "osmosis" contaminants left in the original laminate that the new laminate resin has not cured properly in spots.

In case a) the blister content will be neutral and smell from the solvent. This only calls for sanding, drying and applying of a new coat of water shield.

In case b) the blisters will be acid and have some glycol content. Then a repair must include a total removal of the new laminate. Also the "osmosis" has proceeded deeper into the old laminate.

2.2.2. Hand sized or larger blisters in the new laminate.

When a peeled hull is left to dry in sun or IR heat, the capillary outlets containing acid will be visible as brown spots, often with a running underneath.

Sufficient washing will remove the dry brown acid and create a false impression of a clean surface.  On the contrary the washing opens the capillaries and more acid will evaporate and spread invisibly on the humid surface around each capillary.

The amine type curing agent used for the laminating epoxy is very sensitive to the acid. Therefore a hand sized uncured film will form around most of the capillary outlets when the first of the new laminates is applied.

In case of a recurrence the "osmosis" residues will take the same ways out as before, but instead of forming small coat blisters they will now lift the whole new laminate over the uncured film spots.

The blisters will be low and hand sized or still larger.

2.2.3. No blisters but irregular moisture meter readings.

If a Sovereign meter shows some areas where the moisture is just on the verge to be too high on an otherwise sufficiently dry hull, the Skipper will probably show yellow on scale 1 and red on scale 2 in those and a few more areas.

This means that the "osmosis" proceeds in one or more deeply situated woven roving layers.   Commonly severe de-laminations form between deeper layers, which were allowed to cure too much before the next layer was applied during construction.   In hulls larger than 50´ a combination of both those problems is common.

If more than 4-5 years have passed since the repair, one can be almost sure, that the underwater hull is de-laminated in one or more layers.

Also most of the dry areas will be de-laminated but the "osmosis" fluids have leaked down to the pockets showing the red readings.

2.3. Hulls repaired without relamination.

Here three common types of water shield and/or woven roving in the construction may cause very different signs of a recurring "osmosis.

Very often there will be no visible signs. Only an experienced surveyor or operator equipped with suitable instruments can detect such "osmosis" types before the hull is damaged beyond a possible repair.

2.3.1. Repaired hulls covered with tar epoxy.

If the blisters have been only sanded or blasted away before the covering, the recurring "osmosis" will immediately blister the tar epoxy, just like the gel coat before the repair.

Later the blisters will grow bigger. Sometimes they disappear and instead the "osmosis" fluids will escape down to the bottom parts of the hull and form very large blisters, too shallow to be visible.

If the first repair included a more thoroughly done grinding, blasting or peeling, it may take some time before blisters are formed.   Mostly the blisters are larger than normal already from the start.

It is advisable to test tar epoxy coated hulls by probing along the keel with a blunt instrument for soft spots once a year.

2.3.2. Hulls with elastic water shield epoxy.

The recurring blisters form as described in 2.3.1., but modern epoxy products separate rarely from the laminate in an extent, that the "osmosis" fluids can pass downwards.

2.3.3. Hulls covered with hard water shield epoxy.

This type of epoxy seldom blisters. Instead the recurring "osmosis" first proceeds in the laminate layer underneath the water shield.

The created fluids collect in large areas where the epoxy becomes separated from the laminate.   Due to the non elasticity of the water shield such areas will not be visible or prove soft by probing.

The "osmosis" will spread invisibly into deeper layers and can practically destroy the hull within 5-10 years depending on the rate of condensation on the inside of the hull.

Sometimes, if the laminate contained a lot of acids when the water shield was applied, large blisters as described in 2.2.2. may appear.

2.3.4. "Osmosis" proceeds undetected in woven roving.

In case of one or more layers of woven roving in the laminate, a recurrence "osmosis" may proceed during many years without blistering after any kind of repair.

Due to the large amount of "empty" space in the roving bundles, very little excess acid is formed in the process and therefore most of the cured polyester remains intact.

However a lot of styrene and uncured polyester always exist in the roving meshes and against the laminate layer inside.   In due time the "osmosis" will cause a total de-lamination between those layers without creating any pressure.

The "osmosis" fluids, now a slightly acid solution of glycol, salts and water, will collect in lower parts of the de-lamination and the roving layer.

This type of recurring "osmosis" can only be detected with a moisture meter or by drilling out laminate samples. If the thickness of the laminate exceeds 8 mm, the Skipper moisture meter is the only one we have found suitable for this purpose.

Typical is that the Skipper scale 1 shows green or yellow all over the hull, while scale 2 will suddenly jump to red in some areas.  The borders of such areas are easy to locate as the meter will indicate very big differences when crossing them.   The red areas are often quite large and generally situated in the lower parts of the hull.

Some moisture meters commonly used by the surveyors, do not give any alarming signs of this type of damages if there exists a healthy laminate of more than 3-4 mm on top of them.