PREVENTION AND TREATMENT
Osmosis often occurs when a boat undergoes a noticeable change in temperature, for instance, if it has spent most of its life in the cool seas of northern Europe but is then moved to the warmer waters of the Caribbean or the Mediterranean.
A GRP hull will often be high in moisture content but not necessarily have, or be about to suffer from osmosis, although it’s often an indication of problems to come. Older and thicker solid GRP hulls tend to be able to absorb more water without getting osmosis, but it’s still advisable to store a GRP boat ashore to dry out for a few months each year, where possible.
One way of confirming if blisters are osmotic is by piercing them and testing the liquid inside for acidity using litmus paper. Once osmosis has been diagnosed, however, you will need to determine the extent of the hull damage using a moisture meter.
If it’s only a few blisters over a small area you can usually just dig these out after lifting the boat ashore, wash the cavities well with fresh water and then leave to dry for a few months. If outside you should put a curtain around the repair area. You may also need to apply heat to help draw out the moisture. Better still, get it into a heated boat shed for a few weeks if possible.
Once thoroughly dried out, the cavities can be filled and faired and the area suitably repainted. If, however, large areas or the whole hull is blistered, you’ll need an expert’s opinion. If it’s really bad there’s no other option than a full gelcoat strip followed by systematic treatment of the entire hull below the waterline.