Heidi's Dominatrix, #419, has a rash of low, tiny (3mm) blisters which live only in the gelcoat. We ignore them, and they ignore us. Once, when having the keel slurry-blasted, the operator started to do the hull, too, before discovering the error. This opened the blisters cleanly, revealing the sound laminate below. I filled them with epoxy; a pain.
If I were to go after them, I would have the whole hull slurried, then apply a smear of vinylester filler over the entire surface; rough sand while soft, finish when hard, and cover with vinylester barrier.
We use vinyl bottom paint, which can be removed cleanly with vinyl thinner before hull or keel work is started.
Perhaps there’s no better teacher than experience. Rob wrote us with his own solution:
I had plenty of keel blisters to deal with during my haulout of Hull #62 a couple of weeks ago, but that would probably be another topic.
I had only tiny hull blisters - smaller than pinkie-tip size. The most significant ones were revealed just by sanding the bottom paint in preparation for re-paint. Those I ground out just a little ways with a Dremel tool, and let dry a couple days. I tried cleaning them out with acetone, but that tended to bleed the surrounding bottom-paint into the hole, so I abandoned the acetone.
I tried filling the tiny holes with dedicated, fibered blister-filler from 3M, but it was quite hard to manipulate the fibrous stuff into quarter-inch holes. Instead I used 3M underwater fairing compound.
After sanding the hardened fairing compound, I covered with 3M barrier coating. I found this stuff to be really neat - long working time, thick creamy texture that made it easy to apply and shape with a brush, and easy to sand afterwards (too easy if you are not careful). Some of the fairing compound ended up on top of the surrounding bottom paint which doesn't do much good, but hopefully doesn't do much harm.
Oh, I gave up sanding with an electric sander - the old bottom paint just plugged up my paper quickly. I found that wet sanding by hand with wet-or-dry paper was very effective and made a single sheet of sand paper last a very long time. A great tool for this turned out to be the sanding "block" specifically for sheet-rock mud finish sanding. I bought a plastic one from Orchard Supply. It's long and hard and flat with a foam layer under the sand paper, has a big handle, and gives you lots of options for sanding on the flat face versus sanding a tough spot using the edge. This was a heck of a lot more effective a tool than wood blocks.
I used an industrial-quality chemical breathing mask while preparing and using the chemicals, and a good-quality dust mask when dry-sanding, plus safety goggles. Lungs and eyes are in short supply.
In addition, I talked to Craig Page, Yard Manager at San Francisco Boat Works (next dock to The Ramp) who said that in 14 years he has never seen a S22 with serious blistering. Other Schock boats, such as the Santana 35, are also holding up well--nothing beyond gelcoat blisters.
Craig did mention problems we have grown familiar with (unfortunately)
over the years: mast bridge (compression), iron keel (rust), and rudder post