My boat has wet sailed in fresh water since 1986. It was sprayed originally with a VC Tar barrier coat and VC 17. I have had a few small, superficial blisters on the bottom and along the waterline at the bow. I sand them off and fill them with epoxy and micro baloons and apply VC Tar to the affected area. (Not sure it's still available.) I've filled the ones along the waterline with epoxy with white pigment added. One other boat in our fleet has had blisters. The owner treated them himself--removing the bottom paint, sanding the blisters, filling them and rolling on Gougeon epoxy. This was quite a job and didn't completely solve the problem, so the next owner had the work done professionally--sanding, filling, fairing and applying a new barrier coat. It was not cheap, but I don't think he's had any problems since.

I have always tried to avoid sanding the gelcoat because a fiberglass guru once told me that all that would do was damage the finish and fairness of the bottom when it came out of the mold. He said that it's a mistake to break the surface of gelcoat on the bottom if avoidable. That will cause it to absorb more water and lead to blisters. And most people who sand boat bottoms don't know what they are doing and end up sanding unevenly and making them less fair that they were when they started. Every spring I just sand the VC 17 lightly and touch up any bare spots. When I see any white, I stop sanding and start painting.

Blistering hasn't been discussed much by the class, because most of the class is in San Francisco where nearly all the boats are dry sailed. So, for them, blistering is pretty much a non-issue. At least that's my impression from the hinterlands.