What is Osmosis and How to Deal With It

Osmosis is a degenerative process that occurs inside the fiberglass laminates when water has penetrated thought the gelcoat to the polyester compound. Gelcoat despite it’s excellent properties is not 100% waterproof nor is destined to be waterproof though the life of a boat. Gelcoat loses its impermeability over the years (knocks, natural wear & tear, pores, cracks) and allows water to seep into it progressively.
Once the gelcoat layer has been breached by the sea, the underlying polyester will react chemically with the sea water and decompose by producing acetic acid. Since acetic acid cannot escape back into the water it will slowly build up inside the fiberglass laminates and produce blisters in the skin of the gelcoated surface. Finally these blisters will burst and the liquid inside them will form a greasy brown stain on the hull’s surface.
Osmosis bigger problem is that except from producing blisters in the gelcoat’s surface it also weakens the structural integrity of the boat though the process of osmotic delamination because gelcoat allows sea water to pass though the fiberglass laminates and sea water decomposes the polyester that binds them.
Osmosis treatment with an epoxy base is the best solution for this problem. The first step in osmosis treatment is to scrape the gelcoat surface until its complete removal. Blasting could be also used after the first step for heavily damaged surfaces. Scraping is best done with an mechanical wire brush to eliminate all the gelcoat without damaging the polyester beneath it as well as dry sanding the hull may be required for preparing the surface for the next steps.
The most important step of this process is to let the polyester completely dry, the drying time could take from three months for up to over a year depending on the water volume contained in the polyester, temperature, air moisture and the individual type of the boat. After the boat hull has been thoroughly cleaned and let to dry the first layers of epoxy fillers can be applied and then re-applied until a satisfying result occurs. The epoxy resin is used to form a protective barrier between the boat and the sea water.
Even if blisters have not yet formed on a boat hull this does not indicate that the problem isn’t there, the only way to successfully diagnose an osmosis problem is by using a moisture tool, if the boat has never been epoxy coated it is most likely that such a problem will occur in the future.