OSMOTIC
BLISTERS
Os-mo-sis
(oz mo’sis) n: 1. Physical Chem a. the tendency of a
fluid to pass through a semi-permeable membrane into a solution
where its concentration is lower, thus equalizing the conditions
on either side of the membrane b. the diffusion of fluid through
membranes or porous partitions.*
*From the Random House Dictionary of the English Language.
In 1990
when a boat was hauled for a pre-purchase survey the potential
buyer and both brokers often watched carefully as the hull
broke the surface of the water. The brokers were mesmerized,
certain that this moment would determine the outcome of the
sale and their commission. The potential buyer was rapt with
anticipation, fearing for the worst but hoping for a smooth
blister free bottom. Being neutral with respect to the completion
of the sale reduced the stress for the surveyor, but the tension
always reached a crescendo when the hull bottom was fully
visible. We called this the “broker’s moment”.
A sea
change in perceived significance of blisters has occurred
since then. Gradually, over two decades, many fiberglass boat
owners and potential buyers have come to accept blisters as
a normal condition with fiberglass hulls. While “the
broker’s moment” has lost its drama, discussions
regarding the significance of blister continue, often under
or near boats hauled for survey.
The scientific
data which we have reviewed over the years has generally been
developed by an entity with an interest in the outcome. Resin
manufacturers, repair facilities, repair tool suppliers and
boat builders are among the sources of information we have
reviewed. We are not aware of an independent, unbiased, scientific
study of the root cause of blisters or repair method effectivity.
The opinions expressed in this article come from our experience
in the field, including lengthy discussions with multitudes
of repairers and boat owners. Our opinions are based on examples
of boats that have had various types of repairs performed
due to blisters and on blistered boats which have been left
un-repaired.
To begin
the inspection process for blisters it is not necessary to
haul the vessel. The determination of the existence of blisters
is most easily achieved by a diver. During the age of the
“broker’s moment”, we would often suggest
having a diver inspect the bottom first, and save everyone
some time and the potential buyer some money.
When the
vessel is hauled from the water for survey, it is imperative
that the hull bottom be clean to allow detection of blisters.
Marine growth can hide blisters, particularly small blisters.
A pressure washer can reveal blisters on a hull bottom as
it were a magic wand.
After
the existence of blisters is confirmed visually, probing them
provides more information. Blisters can occur within an exterior
coating and are often found between the anti-fouling paint
and a barrier coat. Blisters can occur within any layer of
laminate, they can be hard or soft, wet or dry. We characterize
blisters by their size, depth, location (external coatings,
below coating, within laminate etc…) number and location.
We feel larger blisters are of greater concern than a large
number of blisters. The depth of the blister is generally
proportionate to its diameter.
Most fiberglass
boats are built of polyester resin. Polyester resin contains
organic components, which will dissolve in water. “Blister
juice” is a sticky, brown fluid with a distinct odor.
It is found in wet blisters which originate within the laminate
and can be found in surface blisters, where the water has
dissolved part of the polyester gelcoat.
The basic
scientific premise is that water molecules diffuse through
the exterior barrier (usually gelcoat) and collect in pockets
below the coatings or within the laminate; thus the term “osmotic
blisters”. Water then dissolves the organic components
of the polyester resin and the resulting pressure pushes the
coatings or laminate outward, forming the blister.
The significance
of blisters is often debated among professionals. The undersigned
is relatively “light” with respect to their significance.
We interface regularly with several respected professionals
who don’t share our opinion. The other side of the spectrum
feels that blisters are cancerous and require eradication
to prevent their certain, gradual and insidious growth. While
we concur that no good can come of blisters and we would prefer
all hulls without them, we feel that they are generally inconsequential
to the ownership, operation and maintenance of a vessel.
We have
never heard of a blister sinking a boat. We have not encountered
multiple blisters connecting and causing large scale delamination.
We are well aware of many examples of large scale delamination,
however none of them have been attributed to osmotic blisters.
We have
found blisters which we feel (and specify in our reports)
require repair, generally they are over 3” in diameter
and well into the laminate.
There
are various repair options for blisters. The depth of the
blisters is often considered when deciding if and how to repair
them. While blisters that originate between external coatings
or the gelcoat and a barrier coat are less significant structurally,
the same repair options apply. A boat owner can do nothing,
repair individual blisters or have “a blister job”
performed on the entire hull bottom.
Repairing
blisters locally consists of grinding the individual blister,
filling the divot (a rare golf analogy in a boat story) and
repainting. This method is suitable for local repairs and
recommended for a boat bottom with a few large blisters.
A “blister
job” involves removing the exterior coatings entirely,
grinding away any dry fibers which remain in the area of the
blisters, allowing the laminate on the hull bottom to dry,
filling individual blisters and recoating the bottom. Often
the bottom is coated with fiberglass material impregnated
with vinylester or epoxy resins. The intent is to provide
a barrier to prevent the osmosis of water, a repetition of
the blister process and to return any strength removed by
the removal of exterior coatings and laminates.
Many blister
repairs have been successful, and in San Diego, Hull Tech
is an experienced company that successfully provides this
service for many boat owners. However, many boats which have
had blister jobs have had the unfortunate experience of more
blisters in the future. At an average “low end”
cost of $225.00 per foot, ranging upward to double that amount,
a blister is a significant financial commitment. As this is
a labor intensive repair, it is not uncommon to have this
job done in areas with less expensive labor. Mexico is an
option and Baja Naval is a skilled facility in Ensenada. If
you decide to have a “blister job”, choose the
vendor wisely. While the value of the vessel will certainly
be increased as a result, the increase will likely not be
equal to the cost of the blister job.
The majority
of boaters who own a boat with blisters choose to do nothing.
We advise them that the blisters are not going to go away
and will likely increase with time. It is uncertain at what
rate they will increase or how large they will ultimately
get, but they are certain to remain. The disclosure of their
existence prior to the haul out for survey is the best way
to diffuse their effect on a sale transaction. An allowance
for blisters is almost certainly less expensive than the cost
of a “blister job”.
In
short it is our opinion that blisters are an unfortunate but
regular occurrence on fiberglass hulls. Their significance
is primarily on the day of sale and rarely significant with
respect to the ownership, operation and maintenance of the
vessel.
