Osmosis is not something that is found only in GRP or Fibreglass boats. It is a process where solutions of different densities are drawn through a permeable material, usually the weaker solution to the stronger solution. It happens in timber as well as fibreglass but is less of a problem. It can and often does occur in fibreglass fresh water tanks, an area often overlooked in surveys.
In laying up fibreglass boats often small pockets of uncured resin get trapped in the laminations. This draws in moisture from the outside or the bilge and as a result more resin is dissolved and so the process goes on. A build up of pressure can cause further delamination to the extent that a structural weakness may develop. The familiar blister on the outside of the hull is a sure sign, but the only real test is to locate the presence of moisture in the laminations. Fortunately there are now instruments that can detect moisture in the laminations that is not noticeable from inside or outside the hull. When slipping your boat a quick run over with a moisture meter will detect trouble spots.
This will also identify those many small blisters caused by trapped air and whilst not osmosis, they will eventually cause trouble. A true osmosis blister, when pierced ejects a pungent acidic liquid, under pressure – so do so only with appropriate eye protection.

Osmosis is curable. If small and isolated spots are found, it is not expensive. However, if moisture is found in many places even though it may have not developed the familiar blisters, early treatment will be less costly in the long run. Even if a hull has no evidence of osmosis, a full treatment with a barrier coat can be good insurance.