Gelcoat blistering problem.

I am refitting a Sabre 27 yacht, designed Alan Hill in 69 & built by Marcon in 71, she had a full epoxy job berlow the waterline around 1980 & was resprayed about the same time.
The epoxy bottom job has stood up well with very low moisture content readings so i have no worries there.
The spray job looks generally very good however!
Trouble is i just moved a fender board she leans against in her cradle & there is a patch of blistering on the topsides about 18" long where water has been trapped against the hull, further aft a vertical patch under a pvc fender has gone the same. This has taken place in just 18 months since she came out of the water.
At first i thought it was the paint but no its gone right through the paint & original gelcoat. Real close blisters about 4mm dia, clear liquid in them. I have had to grind the gel away & it seemed quite soft.
seems like acclerated osmosis but why through a good two pack job & gelcoat?
Any ideas on what is going on would be appreciated.

 

All coating allow some humidity to get through. Do you know what the hull was sprayed with? The term "two pack' is used very loosely and does not mean much.

 

This type of blistering is common and can take place even if below the water line its blister free.

Just like with Gor-Tex, water vapor can migrate through a substance much easier than it can as a liquid. So when water is trapped next to the gel coat for long periods of time like it can be on carpeted bunks, stands, under cushions, under the canvas or when they are shrink wrapped for the winter, blisters can develop in these areas.

The elevated temperatures (compared to actually being in the water) result in high concentrations of water vapor held against the surface resulting in a more rapid blister formation, or even blisters when none will ever develop below the waterline.

 

The paint job is almost certainly 2 pack polyurethane what manufacturer is unknown. Interestingly i was talking today to a moulder i know & he said they had a new boat go like that all over its topsides at a yard he worked at. They put that down to a bad batch of gel & ice damage of all things.
I wonder if thats part of my problem in that the fender board & fender were trapping a layer of water next to the hull & it was the coldest winter here for years.

 

I have seen the same. A decade ago I made trailer molds for dinghies. These were covered in plastic grass. However, some boats suffered from osmosis on the spots where the trailer molds touched. Mostly Lasers, some 420's. Not every boat had that problem.

I guess the quality of the gelcoat and laminate varies badly...

 

Polyurethane paints usually have at least three components(enamel,hardener,reducer) and then whatever additive may be necessary.