It’s the thing that owners of GRP vessels fear most - the dreaded ‘O’ word. Osmosis (also known as ‘Boat Pox’) is a very serious condition. Reports of boats actually sinking because of it are rare indeed, but it can be difficult and expensive to cure so most boat owners greet the sight of blisters in their gelcoat as disastrous. Obviously, any blistering needs to be investigated thoroughly but happily, a blister doesn’t always indicate osmosis and often it is quite easy to cure. So how do you tell the difference?

Osmotic blistering
Blisters caused by osmosis will be found around or under the waterline. They may be small pin pricks or large swellings as big as the palm of your hand. Puncture an osmotic blister and there will be fluid behind it. Fluid in itself is cause enough for concern but if the fluid has a pungent vinegar smell, the indications are towards osmosis. The smell comes from acetic acid, due to the hydrolysation of the emulsion used as a binding agent in the manufacture of chopped strand mat.
It would be unusual to find osmotic blisters on a boat until she had been immersed for several seasons but you can confirm your diagnosis by using PH paper. A PH value of less than 7.0 indicates acid. Readings of between 5.5 and 3.0 are typical when water and binding emulsion have joined forces - and this would be reason enough to call in the experts.

Non-osmotic blistering
This can be found in the same areas as osmotic blisters, in clusters along or under the waterline. They tend to be small pin pricks and are rarely large. They also tend to be much tougher than osmosis blisters and when you break into them, they are dry and free of any pungent smells.
They can occur for two reasons. Either air was trapped between the gelcoat and the inner mat during manufacture (and the voids have enlarged due to temperature changes), or there is a fault in the gelcoat due to the absorption of water into the pigment or extenders when it was manufactured.
Unlike osmosis, this problem is more likely to surface within a few months of launching, and the cure is quite simple. Strip off the antifouling, degrease and sand the gelcoat, fill the holes with epoxy and then apply an epoxy paint and re-antifoul. It is a serious job and it needs doing - but it is not a disaster.