I remember from an episode on ShipshapeTV the exact same problem he tackled on a Bertram 31. The blisters were small, hard and fluid filled (smelly). He actually stripped the whole gelcoat off the bottom and then ground open all the blisters that were still closed. Then they had to let the hull sit for a while while the blisters dried out before sealing them and re-gelcoating the bottom. Apparently, no matter what you do, it's a long, tedious process.
