but I think you are wrong. In fact, IIRC, the process can be faster in fresh water due to the greater density differential between the water outside the hull (fresh water being less dense than salt) and the fluid in the blisters.
Serious Osmosis will result in thousands of blisters, far too much to hide with paint, or in some cases, several extremely large blisters (the size of dinner plates), so again, v hard to cover up......
If fresh water has been sitting inside for some time you can expect to get a high reading from a moisture meter. I was caught by this when selling my last boat. A cockpit locker filled with rainwater over the winter which then overflowed into the hull, resulting in a surveyor's "high mosture reading" observation which in turn was translated as "hull riddled with osmosis". Complete bllx but it certainly dropped the price I got.
I used to run a sailing centre on a reservoir where the water catchment area was very peaty.
We used GRP Wayfarers as the bulk carriers and I bought 3 from Porters when he started building them. They all had to go back after a season on their moorings as huge blisters had started appearing. A surveyors report commented that the high Ph content in the water had almost certainly contributed to the osmosis.
