With soft ball size blisters you'd have to look at them anyway so go
ahead and cut them open. If they are not into the laminate, I'd say no
structural problem.
One web site discusses actual measurements on blistered LAMINATE and
found that the tensile strength was hardly affected but they noted that
flexure might be a problem causing the hull to "oil can" if the
LAMINATE was blistered (a rare condition).
One potential "solution" I have never heard anything about for people
who are concerned about possible structural problems but who
cannot/will not peel the gel and skin coat is to add a layer of
epoxyand glass INSIDE the boat. For a small lower value boat like mine
(28' worth maybe $15000), this might alleviate some structural worries
without spending a huge fraction of the boats worth on a gelcoat peel
that probably will not solve the problem.
Even with a worst case scenario, for a normally built laminate, I do
not think anyone can point to any accident caused by structural failure
due to osmotic blistering. Just in the interest of gathering data, I
invite anybody who has real knowledge of a real structural failure due
to osmotic blistering in a conventional laminate to post such info.
I tend to be a skeptic about blistering woes partially because of a
Practical Sailor article years ago where they decried the longetivity
of fiberglass boats as depressing the market for new boats. At the
same time, they hailed the blistering problem as a possible saviour of
the boatbuilding industry. In other words, there are many people with
a vested interest in creating problems where they may not exist.
My experience with boat yards has never been good although this may be
a product of living in a place with no good ones. I have found that
generally I will do a better job than they will and that they can be
expected to take short cuts. In blister repair, all short cuts will
cause failure of the repair.
Pascoe noted that even some of the best known highest quality builders
had severe blister problems so it isnt confined to low cost or low
quality boats.
In summation, I would not reject a boat with blisters but I would get a
surveyors opinion and use the blisters to bargain.