What is osmosis and how can it be treated or prevented?

Over 30 years ago, boats were made from a material called fiberglass. This material was popular for its high strength and long lasting durability. It was also alleged to be low maintenance. Fiberglass hull is made up of two separate layers. Both layers of fiberglass laminates are semi permeable, meaning that water can pass through the outer layers.

Osmosis is what happens when moisture seeps into the fiberglass laminate, through either layer. The moisture then gets into the fiberglass and fills all of the voids, therefore becoming trapped. It then produces acidic blister fluids, as the water between both layers puts pressure on the gel coat.

This affects the outward appearance of the boat as well as the overall performance of the vessel. As this only happens through moisture, the appearance of the boat can remain unchanged, as blistering only occurs below the water line.

This is why osmosis treatment is vital because if the blisters are left and are not checked they will deepen inside the fiberglass laminate and will eventually have an effect on the boat’s structural integrity. It does not matter how good the quality of the construction is, osmosis blistering will always take place in most boats at some time or another.

The method of osmosis treatment is mainly required by a professional. The boat will need to be dried using de-humidifiers along with infra-red lights. It will then be sheaved using a multi bi-axial mat as well as a special resin. After this process it will be given a further 5 layers of gel-protective solvent free epoxy. Following this is another filler coat and then the last few antifoul coats.

The temperature during this whole process is important when preventing osmosis, as a high temperature will help the process whereas a low temperature will stop the hardening and the whole procedure will have to be carried out again.