The rule? Don’t ignore them, catch them at an early stage
If there’s one word that can instill fear in the heart of the toughest mariner, it’s osmosis. The discovery of osmotic blistering in a glass-reinforced plastic (GRP, or fiberglass) hull often carries the same connotation as a diagnosis of a terminal disease. Boatowners who are advised that the love of their life is suffering from the dreaded boat pox fear the worst. Sometimes their fears are justified because serious cases of osmosis are expensive to cure; however, in many cases, a slight rash of blisters can be no more than a minor nuisance, a problem that you can easily fix yourself.Because the causes of osmosis are so many and so varied, so too are the results and the cures. There is just one rule that applies to all cases: If you ignore it, it will get worse.Most blisters start in the outer 1/10 inch (2.5 mm) of the GRP. One theory is that they’re caused by the migration of water vapor through the gelcoat into the laminate, where it reacts with certain chemicals and causes pressure.Small shallow blisters are more of a cosmetic problem than anything else. They can be drilled out, drained, allowed to dry, filled with epoxy paste, sanded, and sealed. The hull should then be coated with a waterproof barrier and, with any luck, you’ll have no more osmosis problems.Deep blisters that penetrate more than one fifth of the laminate may be structurally serious, leading to delamination. Serious blistering calls for expert attention, and may involve stripping of the gelcoat to allow the laminate to dry out. An experienced surveyor will be able to advise you.Peeling off the gelcoat and affected laminates and rebuilding with vinylester resin and top coating is an expensive business. It could cost 25 percent or more of the value of a fairly new boat and as much as 75 percent of the value of an older one; therefore, check your hull frequently and catch any blistering at an early stage.See also Barrier Coats..
