[akelley2]

I hadn''t heard that freshwater causes more blisters than salt, but it makes sense from a chemical point of view. Osmosis is the flow of water from an area of high concentration to an area of low water concentration. In polyester resin, there is no water, so the osmotic pressure (tendency of water to flow into it) is high. Saltwater is less concentrated water (more salt!) so the osmotic pressure is lower between saltwater and resin than freshwater and resin.

All that aside, I do not know whether the typically more acidic freshwater is harder on gelcoat than the slightly alkaline saltwater.