How do I recognize osmosis?

The main symptom – blisters

Blisters are the most common warning sign and if identified should be followed up with immediate professional examination. Blisters can vary from small pinhead blisters, to areas as large as the palm of a hand. The presence of any fluid behind a blister indicates a potential problem. If the fluid has a pungent, vinegary odor or feels greasy or sticky when rubbed between the thumb and forefinger, there is a high probability of osmosis. Before any treatment is carried out, you need to establish what has caused the problem. We recommend that you seek the advice of a professional surveyor. Some blisters occur for reasons other than osmosis. They are often evident as a rash of small pinhead blisters or swellings, either locally (often around the water-line) or over the entire underwater area. These blisters are hard and difficult to break and when broken open will be dry, with no odor evident. The likely cause is air voids. This is not a serious problem, but hull moisture levels should be checked before commencement of any remedial treatment.

Other warning signs to look for


Prominent fibers: Seen protruding beneath or through the gelcoat and can cause ‘wicking’ where water is drawn into the hull by capillary action.


Star crazing: This effect can occur where the gelcoat is brittle. Fine cracks usually form due to severe flexing or impact damage, allowing water to seep into the laminate.


Pinholes: Tiny bubbles present in the gelcoat reduce its effectiveness and promote rapid water absorption.


Undercuring of the gelcoat: Incorrect mixing or application in unsuitable conditions can cause failure to cure properly. This results in porosity and may lead to water ingress.