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Diagnosing Osmosis

Planning the  Work


Inspecting & Drying

Materials Required

Epoxy Application

We recommend Advanced Osmosis Technologies to provide you with an expert osmosis diagnosis and repair service

Planning the work

PREVENTION OF OSMOSIS New craft and those with a low moisture reading and showing no signs of osmosis should be degreased with SYNSOL 100 solvent, sanded to a mat finish with a dual action sander, and given four coats of SAFEGUARD EA or TSF followed by one coat of EASY FAIR SEU. The EASY FAIR SEU should be sanded to a smooth finish before application of the antifouling.

TREATING OSMOSIS  The treatment of osmosis depends on the severity of the damage to the laminate. The primary defence against the continued development of osmosis is the prevention of moisture entering the laminate. SAFEGUARD’s unique  combination of low solvent or solvent-free epoxy resin and solid ceramic micro-beads gives the toughest protection available. SAFEGUARD’s properties make it suitable for DIY application and so minimises the cost of treatment.

The three main stages of osmosis and its treatment are detailed below.


Osmosis is indicated by the presence of high moisture levels in the laminate but there is no visible damage to the laminate or gel-coat.


Remove existing bottom paint over the area to be treated.


Abrade the gel coat to a matt finish.


Dry the hull to a moisture reading of less than 5%.


Degrease with SYNSOL 50 solvent.


Apply 5 coats of SAFEGUARD EA or TSF


Apply 1 coat of EASY FAIR SEU.


Sand smooth and apply antifouling


A few blisters are found or blistering is limited to one or more specific areas. This is generally  indicative of minor, localised defects in the original laminate.

Experience indicates that localised treatment of these areas, as outlined under stage three, and applying SAFEGUARD EA or TSF over the whole of the hull will provide protection against further damage. Always make sure the laminate is dry and has been checked with a moisture meter before application of SAFEGUARD coatings.

Chiseling, drilling or digging out of individual blisters will not be an effective treatment because the laminate will have degraded in the area around the blisters.


If osmosis has advanced to the stage where extensive blistering has occurred over large areas of the  hull and damage to the underlying lay-up is apparent it will require complete osmosis treatment.


Remove the old gel coat with a gel plane machine.


Blast the laminate to etch the surface and remove soft areas.


Inspect the laminate to ensure that no damaged areas remain.


Remove any remaining damaged laminate


Pressure wash several times preferably with hot water to remove salts and contamination.


Dry the laminate to below 5% moisture.


Replace any damaged laminate.


Prime the laminate with SAFEGUARD LVP.


Fair the laminate with EASY FAIR filler and sand to a smooth finish.


Apply of 5 coats of SAFEGUARD TSF or EA.


Apply of 1 coat of EASY FAIR SEU.


Sand to a smooth finish.


Apply antifouling

Osmosis in glass fibre boats is indicated by blisters in the gel coat


SAFEGUARD coatings, as with most other Reactive Resins epoxy products, do not require the hull to be under cover for proper application. The hardeners are moisture tolerant and they enable the epoxy to be applied in a wide range of temperatures in high humidity without blooming.

Work must be carried out in dry conditions. If the vessel is indoors work may proceed independently of the weather.

An economical alternative is to ‘tent’ the hull with plastic sheeting after removing the gel-coat. Adequate ventilation must be provided to avoid condensation. Tenting will avoid having to wait until the hull has dried after rain.

Under no circumstances use a heater with an open flame such a diesel, kerosene or LPG space heaters. All will produce CO2 which will react with the epoxy hardener to form a carbonate, sometimes called ‘bloom or blush’. This will seriously reduce inter-coat adhesion.