'Plastic Worm' and (wrongly) aka 'Osmosis'
The damage from “osmosis” is not caused by excessive water permeation. There are no osmotic forces involved as the gelcoat is an absorbing material which can never act as an osmotic membrane.
Instead it is the process of hydrolysis, which creates the water soluble corrosive products which in turn create the familiar cavities (blisters in the gelcoat). Hydrolysis is the general term given to the reaction of any material with water, for example organic compounds like polyester resin, itself the main constituent of GRP laminates.
Once the cavities have formed, then excess water will enter giving rise to high moisture readings. This process may normally be slow, but the presence of free acids or alkalis will greatly accelerate it.
Osmosis blisters are not only a cosmetic problem. They are the visible sign that the hydrolysis of the polyester has affected at least one laminate layer.By the time the first blister shows, hydrolysed alkali products will have reduced the bond between fibre-glass and polyester deep in the laminate below the water line - in other words your plastic tub will start to delaminate - not something you want to happen at sea.
So, the process of repair is time consuming and expensive as the gelcoat has to be 'peeled' off completely using a special plane, then the salts of the hydrolosis have to be continually washed off at periodic intervals as they tend to rise to the surface again. Following this a period of drying using both infra red lamps and a new system called 'Hot-Vac' to completely dry out the whole structure.
Once the moisture readings fall to an acceptable level, (between 1 and 3% - mine is currently at an average of 22% - a serious level) then the drying and cleaning process is complete and we are probably talking up to 2months, then the gelcoat is replaced and the hull double epoxied to the water line.
With boats, it's not a question of 'if' a boat suffers from this process, it's 'when' and the earlier the boat the more prone it will be. Boats kept in fresh water are more likely to suffer.
That's it! The sum total of my knowledge and I have had to pay dearly to acquire it!
Plastic (GRP) is not the wonder material for boats that was first trumpeted. Steel rusts & aluminium corrodes and makes for an always damp interior unless comprehensively lined so that really only leaves wood.
Wood is good
