When my boat was hauled after Sandy wrecked our marina, I found hundreds if not thousands of small blisters all over the hull of my W32.
After letting the boat dry out all winter, I scraped and sanded off all the old antifoul paint. I dug all the blisters out with a dremel. There were maybe 10 or so deeper blisters, many near the prop that went a few layers into glass. After opening them all up, I let the boat sit another month, periodically washing the hull with Dawn and rinsing. This supposedly helps to dry the hull.
I repaired the deeper blisters with proper fiberglass and epoxy patches. The smaller gelcoat blisters I filled with 3M "Marine Premium Filler"—a vinyl ester putty which is far easier to work with and fair than epoxy, but also rated for below the waterline.
In the hopes of preventing more blisters, I barrier coated with five coats of Interlux Interprotect 2000e, and finally antifoul paint (Pettit Trinidad SR). The hull looked amazing when I was done. I only regret I could have gotten my act together to do the Dave King hull mod.
My boat was left in the water for two winters, then pulled this year. The hull is still the best looking part of the boat.
It was a very time consuming project, but in the end worth it for me.