In 1990 I repaired 8 blisters on my sailboat. They were 3/4" - 1 3/4" in diameter. There was a fiberglass
specialist in the yard and said he would work
on a time and material basis and let me do the basic stuff as he advised.
We sanded the blisters open and down to clean laminate, with a tapered edge. Each resulting crater was about 5" in diameter. We used about a 40 grit disc. We let it dry out for 5 weeks (in Florida) he checked the hull every week with a moisture meter.
He told me to wipe each crater down with acetone a few times each day.
when doing the repair, he had me cut a bunch of different sizes of fiberglass material. I think it was mat. He coated the crater with epoxy
and laid in a piece of fiberglass, rolled it and repeated, using a larger piece each time, until the crater was filled.
The next day he came by and put on a fairing coat, and it was done.
Of course a week later when we moved a stand to paint
under it, we found another blister we missed!! Damn!
I had him look at it and told him we had to be leaving soon. He told me to grind it out and hit it with acetone every few hours, and 2 day later he would mix up some "hot" epoxy and repair as he did with the others. he said it would probiblly be ok.
We cruised full time for 6 more years, and then 2- 4 month cruises after that, and none of the blisters ever came back. The boat
did develope maybe 2 or 3 new blisters, in the following years.
I would never consider removing the gelcoat
and doing an epoxy bottom, unless it was real bad.
I know a friend who has had his Hans Christian 41 botton pealed and done 3 times!!! True story.