Osmosis

Despite all the technical progress of the last several years, polysters, which constitute most boat hulls, are not totally waterproof. It isn't a problem, except when water particles manage to inflitrate the empty spaces in the hull as balls of air or badly reinforced areas.

Upon contact with soluble materials (constituted by the additives of the stratified matrix or present in the components of the binding agent between the fibres and the resin), water is charged with elements. The concentration of elements increases with the dilution of the plastic components and then osmosis appears. The volume of liquid contained in the cavities increases until creating a pressure able to change the surface and generate a blister. The blisters resulted from osmosis are recognisable by the pressure that reigns inside and the acidity of the liquid (acidic acid at pH <6) emiting a strong smell of vinegar.

A lot of different causes can create the blisters in the hull. The dimensions of the blisters can reach several millimetres to several centimetres, their distribution either homogeneous or localised. The blisters can appear at three different levels of thickness in the planking :

Between the antifouling paint and the gel-coat, they are the result of a bad preparation of the hull : a humid surface, impurity deposits, bad mixture of components, too much or inappropriate solvents, undercoats of primer incompatible the one with the other.

Inside the polymer, they are the consequence of water absorpsion caused by the capillarity action along the fibreglass. The principal reasons here are : a too high proportion of dilution of the stratified resin softening the gel-coat, bad conditions for hydrometry and temperature, too short dry-up period, wrong check of the polymerization and of the glass/resin proportion, no post-heating allowing to release the remaining styrene in the polymer.

One on the major causes for osmosis however is the lack of daily and preventive maintenance of the hull. The natural erosion of a gel-coat can start to be significant at the beginning of the 5th year. The gel-coat isn't designed to undergo mechanical aggressions as high pressure cleaning or scratching by a metal spatula which accelerate the degradation. It is therefore important to remove regularly the antifouling crusts and to carry out a visual check-up of the hull, searching for any damage, break-outs, cracks, chalking or micro bubbles. After a complete drying, the repair of this damage must be carried out without delay and exclusively with the help of an epoxy system to fill in the damage and/ or to follow a cycle of polyurethene.

Our advice :

• If you buy a more than 5 years-old boat, have an osmosis diagnosis of your hull done.
• The absence of visual osmosis indicators (blister), does not mean that the polyster isn't abnormally damp.
• A boat with osmosis doesn't sink but it is heavier and loses its value. Don't wait for this phenomena to happen, have a permanent preventive action.
• Apply an epoxy resin waterproof enough to improve the preventive protection of your hull.
• In case of a proven osmosis, you need to use the technique of plaining the gel coat. Removing it by sandblasting or waterblasting can take off layers of the gel-coat.
• The condition of the bottom of the boat is something you need to watch closely : it is necessary to avoid the stagnation of dripping water due to condensation or inflitrations (stern gland etc).
• Avoid the silicon-based polish, that can possibly damage the gel-coat. It is better to use a polymer polish.
• Avoid antifouling paints containing choloroprene, which are strongly suspected of degrading the gel-coat.
• Avoid high pressure washing of the layers of antifouling.