I wanted to grind off the bottom paint as it was in terrible shape for a barrier coat and new bottom paint.. Got started and noticed many tiny blisters and quite a few big ones. When I starter on those, the mat under the gel coat was bad and I started peeling that off. As I progressed, I found more and more bad spots. Wound up grinding all the gel and matt off down to the roving. Where the roving was soft or oozing water, I kept going. Fortunately, most spots were pretty small areas but much larger than the evident blisters. That's why I say blister aren't usually a problem, unless they are signaling a bigger problem which blister repair doesn't solve.
The only big area was a pain to layup by myself...but I got it done and sheathed the whole hull in a layer of cloth and good layers of Interprotect 2000.
2 Subsequent surveyors were impressed with the repair (though not cosmetically perfect) and not one blister after12 years.
I have worked on plenty of different hulls and can say there are almost always more problems with at least parts of them that go undetected till you start tearing into them, often because of other issues.