In this part of the country (Chesapeake Bay, VA) the prevailing wisdom and experts (surveyors and working boatyards, builders) is that all fiberglass boats will blister at some point. I recently bought a 1983 Sabre and she had a dozen or so "blisters" on one side, rear of the keel.
Blisters as defined here are small pockets containing moisture between the gelcoat and the fiberglass roving. One would even say smaller than a silver dollar. Our surveyor, and he is one of the best in the area, as well as the yard manager suggested that immediately upon hauling, power wash and as the water runs off, you will see the blisters. Mark them with a pencil (circle them) as being able to see them clearly will go away fairly quickly.
Then, using a sharp putty knife cut them and they should "bleed". Do NOT cut in to the fiberlglass. To do repairs correctly, it is suggested to let the blisters dry completely remove the "loose" gelcoat and repair. You may even have to recut them and let them dry for several weeks or a month or so.
Here they do not recommend "barrier coat or expoxy bottoms" as the feeling is that this just traps the moisture and will eventually lead to delamination.
Hope this helps.