Osmosis is a phenomenon of natural aging polyester. It develops slowly and may appear as small blisters (size of a pinhead) after a few years. All polyester boats may facing to this phenomenon.



Water is soaking in the polyester, and will destroy the resin.The boat becomes soft and is likely to decompose.

Osmosis is directly related to the components that go into the manufacture of a polyester hull: the acid and the alcohol. This mixture which is added to the fibreglass may change from viscous to solid state with the help of a catalyser. Upon completion of this mixture, sometimes strands of glass fibers are mixed into the resin without being fully impregnated with the catalyst or the chemical reaction is incomplete. This creates small cavities in the structure of the hull, filled with a highly concentrated liquid, composed of acid and impurities. This liquid is neutral unless it is through contact with water.

Natural osmosis is the imbalance between two liquids of different concentrations separated by a semipermeable membrane. The slightest concentrated liquid passes through the membrane to go to the most concentrated.

On a boat, there is one light-concentrated liquid (water) and one high-concentrated liquid (in the boat’s structure) which are seperated by a semi-permeable membrane : the gelcoat. The concentrated liquid contained in the hull (which is in contact with water that has passed through the gelcoat) makes chemical reaction (hydrolysis) and will be changed in acetic acid, whilst increasing the volume. With a modern good quality,waterproof gelcoat, the water, which has penetrated the gelcoat by osmosis, and which is likewise converted into acetic acid (in reaction with the liquid contained by the polyester) cannot leave. This increases the volume appearing as blisters.

The blisters are filled with acetic acid identifiable by their distinctive vinegary smell (it comes out when pierced.) This also explains why, with an old porous gelcoat, there are no blisters, hydrolysis still occurs, the acid is still created but, instead of causing blisters, it passes back through the gelcoat.


  • Blisters : Blisters can vary from small pinhead blisters, to areas as large as the palm of a hand. The presence of any fluid behind a blister indicates a potential problem.
  • Star crazing : This effect can occur where the gelcoat is brittle. Fine cracks usually form due to severe flexing or impact damage, allowing water to seep into the laminate.
  • Pinholes : Tiny bubbles present in the gelcoat reduce its effectiveness and promote rapid water absorption.


Preventing osmosis consists in creation of a waterproof barrier between the resin and the water. For this, we use an epoxy resin. On a new boat, it is possible to provide this treatment before the first launch. On a boat that has already sailed, ensure that the hull to be completely dried and not to trap moisture behind the epoxy barrier. On a boat which contains moisture, it is necessary to remove the gelcoat (by planing), to dry the boat (sometimes several months) and then redo it including the epoxy barrier.


  2 coats                 1 coat                                           Filling                              4 - 6 coats               2 coats 

       +              /     +          +     

  • Substrate preparation : Gel plane peeling or polyester grit blasting to abrade surface prior to treatment. Rinsing with fresh water & drying of laminate. Repeat it to ensure removal of any unwanted (salt, soluble residue) and moisture. Drying of laminate with adapted system depending on location and season : ventilation, air flow. Keep readings of Temperature/Humidity system records by checking systematically above and below the waterline. Before starting the treatment, be sure humidity rate of the hull is no greater than 5% to the rate above the waterline (Tramex 2 to 10 /scale1).
  • Sealing the substrate with IMPREGARD (solvent-free epoxy resin for impregnation)  : Apply 1st thick layer by brush or roller. Once dried : Sand the surface with grit P80 to lop the ends of fibreglass which go beyond the surface by “wick effect”. Apply 2nd layer by roller and ensure the fibre's are fully coated in resin.
  • Fairing with EPOXYGARD (protective epoxy primer) : Apply 1st layer.
  • Filling with epoxy fillers (WATERTIGHT or BLUE FILLER) : Application of filler is optional : depending on importance of surface imperfection.
  • Protection & finishing with EPOXYGARD (protective epoxy primer) : Apply 4 - 6 layers (depending on thickness). Respect overcoating time between layers to allow good evaporation of solvents.
  • Apply 2 layers of nautix antifouling.