Osmosis Types

Blister types:
Blisters are often described as types. The British Plastics Federation (BFA) published a booklet in 1984, ‘Repairs to Blisters in Glass Fibre Hulls’. Although now a little outdated, still holds important and relevant information.

BFA categorized blisters into three types and two sub-types, with a description of the characteristics of each blister.
Blisters range from small pin heads, raised domes of 5-15mm and to large broad blisters of 50mm or more. The latter blisters and some of the domed blisters usually hold quantities of acidic liquid.
These acids are formed by the breakdown of the binding agents used in the Chopped Strand Matting (CSM) and can usually be identified by the characteristic vinegary odour.

Interestingly enough, BFA also states that the presence of blistering through osmosis does not necessarily mean that the boat hull is in imminent danger of structural failure.

Repairs:
To fully determine the type of blister and the condition of the laminate, invasive testing needs to be undertaken. This may involve drilling a shallow hole into the laminate or a more aggressive grinding to expose the laminate. Running a moisture meter over the hull without inspecting the laminate is not a satisfactory method of determining laminate condition.

For the DIY boat owner it is possible to deal with small blisters but some basic rules need to be followed. One of the hurdles is to remove residual acids and contaminants before repair work is undertaken. This can be almost impossible to achieve unless the boat is undercover and thorough removal of contaminants and drying of the hull is done.
Drying out a hull without removing any contaminants will not cure the problem.

Grinding out blisters and filling voids with polyester bog or even epoxy before flushing out the laminate and drying, will not work as a repair method. Yet I have seen owners applying ‘barrier coats’ and smearing resin and fillers into hulls without proper preparation.
Paint system ‘barrier coats’ can accelerate blistering if moisture and contaminants are trapped within the laminate. Applying such systems without fully understanding the condition of the laminate will often result in added expense and ultimate failure.
There are some proven methods for blister repair including the ‘Hotvac’ system.
I recommend that if you are serious about repairing hull blisters to a high standard, then use repair yards that offer warranties on their work.