Blisters on fiberglass yachts are sort of like Bobsprit.
They show up only after the work has been done.

Extensive study on blisters has shown the following:

a.. It is not possible to effectively solve the blistering problem of a hull that is water permeable.
b.. Water permeability of a laminate is not the function of a resin alone, but how well the glass fibers are saturated or wetted
out with resin. If a hull is permeable from the outside, its also permeable from the inside. What is the point of recoating the
outside when the inside is also exposed to water?
c.. Blistering on above the waterline structures proves that immersion is not necessary to cause blisters.
d.. We learned from the Uniflite class action suit in the early 1980's that continuous strands of glass fiber are capable of
conducting water along their entire length by means of the capillary effect. For example, if a roll of roving is laid out from one
end of the hull to the other, and the wet out is not good or complete, those fibers can wick water along the entire length of the
sheet of glass.
e.. We also learned that engine vibration transmitted to the hull is one of the major means by which water is transmitted through
unsaturated strands. Capillary effect alone is not responsible.
f.. In a four year casual study that involved examining every hull that I ran across that had the outer coatings removed
(involving hundreds of boats), the lack of complete wet out was appallingly bad in well over 50% of all boats that I looked at.
There is a direct correlation between low quality resins and poor wet out on blistering.
g.. The use of chopped strand mat as a skinout layer to prevent telegraphing of weave patterns through the gelcoat is a major
source of the problem of water absorption of the hull. This is because mat does not wet out well. Further, because the fibers are
short, there are millions more exposed ends of fiber bundles capable of wicking and conducting water through the laminate. Heavy
layers of mat are very hard to fully saturate with resin. It is also responsible for causing millions of small voids that ultimately
fill with water. Its almost like a wood boat that has millions of tiny worm holes.
h.. Exposed fibers on the inside of the hull are also responsible for wicking water into the laminate. There is a definite
correlation between where blisters most commonly occur and where bilge water lays within a hull. There is also a correlation of the
predominance of blisters and the edges of sheets of glass fabric.
i.. Major blistering problems are often related to bonding failures of both gelcoats and skin out mats. I was not aware of this
until about a year ago, when, one day, I watched a blister repair contractor stripping the gelcoat from a hull. Not only was the
gelcoat coming off, but the entire skin out mat was peeling off (see actual photo of this boat). On closer examination I found that
major areas of the skin out mat had never achieved bonding and could be peeled off by hand. As I continue to examine boats for this
condition, I am finding more and more of them.
j.. Examining the process that yards use to remove the blisters, it is found that most keep grinding away until they stop seeing
any evidence of delamination. The problem that they often run into is that the delamination never stops. The nearby photos reveals
some of the conditions I have found in the vast majority of all blisters that I have examined (thousands). That is that most
blisters involve ply separations that seemingly never ends. They grind and grind and grind, but there's always separation around the
circumference of the blister.
k.. When ply separations or incomplete bonding exists, blistering is a problem that cannot be solved. That's because the void
areas are going to fill up with water all over the bottom. Repair the blisters and they will reappear because its not possible to
keep water out of a hull that's constantly immersed in water.

I think the above is good information. I particularly like the
part where it says engine vibration is a major cause of wicking
and blistering. All you diesel lovers take heed. I've seen boats
with big inboard diesels literally shake all over.

S.Simon

references: http://www.yachtsurvey.com/moreonblisters.html.htm