* By the way, blisters tend to be non re occurring... - That's true if the repairs are done correctly, the most important part being adequate drying time followed by a correctly applied barrier coat with proper film thickness
There are dozens of horror story around the web about expensive 'professional' yard blister repairs where new blisters formed a year or two later... because yards rush the jobs and don't let the hull dry out enough; it isn't new water molecules coming in through the barrier coat, its the water that was already in the laminate that got sealed back in under the new paint.
Everyone, from the prospective novice boat buyer to the so called "professionals" seem to have their own idea as to what the actual severity of any given blister is, when in fact its pretty straight forward to those of us who have studied them and done numerous blister repairs, and learned what works and what doesnt.... and I can only say this after utilizing and gleaning information from all the scientific studies and work that has been done on the problem in the past 40 years.
within the last 20 years, the largest percentage of so called "professional" jobs that are expensive and have failed, were more than likely done in a yard by a minimum wage worker who had no clue, didnt care about the boat or its owner, but was being pushed to get the job done by someone who was only slightly more qualified, when all the worker wanted to do was to be anywhere else in the world, as long as it wasnt at work making fiberglass dust..... just because the yard charges a lot for the work being done doesnt mean the employee doing the work is qualified to do it, or that his supervisor is even qualified.
drying time (and patience) is always your friend when planning to barrier coat...
large blisters, quarter sized and bigger should
normally always be ground and filled.... (there are exceptions)
the small fingernail sized blisters that sometimes will cover the entire bottom, or large area of the bottom, and makes the bottom look pebbly, can almost always be dried out to where they will recede back down... sanded over a little and they will never show again until the gelcoat has a chance to suck up water again.
attempting to perforate all the blisters is good, but only so they can bleed out and speed drying time. some of these small blisters will inevitably be missed, but with adequate drying time, it wont matter..
the reason that it can recess back down is not a direct result of the water behind it being removed, but because the gelcoat DOES absorb water, and when it does, it wants to swell a bit, (like a sponge, piece of wood or other object that can absorb water) so, where the water can get behind the gelcoat in the tiniest recess, it will absorb into the surrounding polyester and weaken even more an already weak adhesion, to allow the gelcoat to lift away from the resin laminate behind it, and once it begins to come loose, the swelling of the gelcoat finds relief in pushing outward and doming/blistering, which pulls at the edges of the weak adhesion within and causing the blister it to grow larger until it gets big enough that the blister has enough surface area to absorb all the swelling and lifting within its domed surface, or the adhesion is so strong that it can lift away anymore... due to the doming action allowing the gelcoat room to swell, its extremely rare to see a blister as large as ones hand.
the mil thickness of the gelcoat at a given area, usually has a direct effect on the size that the blister can grow to.
thick gelcoat will not have fingernail sized blisters, and thin gelcoat will rarely have large blisters.
nearly all blisters are NOT a hull threatening problem, but they are ugly, can slow down a racing boat and likely to cause the resale value of the afflicted boat to plummet.
But when one opens a large blister and finds a large dry area of resin starved laminated behind the gel coat, that goes deep, then there is a problem and time to break out the big money....this is a structural problem.
and I will also agree on a few other points made...
if ones intent is to seal the moisture out forever, take the time and precautions necessary to make sure you dont seal any in....
the bottom of the boat can take some severe abuse with grinding discs and sanders, as long as the material taken away is replaced with something equally as strong or beneficial....
the less disruptive you can be to the bottom of the boat and still do a quality repair, the less time it will take to complete the task and the easier it will be..