is an attempt to demystify and clarify the fiberglass blister problem.
observations by Hartoft Marine Survey, Ltd. indicate that approximately
70% of the fiberglass vessels in the Chesapeake Bay area when inspected
have fiberglass blisters to varying degrees of severity. This figure
has recently been decreasing probably due to repairs having already
been made. It is believed that approximately 95% or more of all fiberglass
vessels in this area have, had, or will have, fiberglass blisters. Further
observation shows that bottom laminate constructed with the commonly
used orthophalic resins has a life expectancy of around 30 to 35 years
before deterioration due to blistering and resin damage has structurally
weakened the bottom laminate. This deterioration can often be observed
as flexing of the bottom laminate when hand pressure is applied even
on boats in the 40' to 50' range.
have recently changed the resin types used in construction to higher
quality resins (to isophalic or even better vinyl ester resins) in all
or some of the outer layers of their vessels. Depending on resin type
and application some of the comments above and below may not apply.
Please consult with the builder for the exact specifications.
commonly referred to as a "Fiberglass Boat" is more correctly
a "Fiberglass Reinforced Polyester Resin Boat". The hull skin
is normally built in a female mold. First gelcoat is sprayed in the
mold and then successive layers of different types of fiberglass (cloth,
matte, roving, etc.) is layed into the mold one at a time and wetted
out with polyester resin forming a hull consisting of many layers (or
laminates) of fiberglass bound together with the resin.
blistering is caused by one or more factors such as resin type, contamination
of materials, trapped gases, built-in voids, poor wetting out of laminate,
incorrect humidity or temperature and dry layup. Osmotic fiberglass
blistering is a process which depends on the temperature of, and exposure
time to, the water. Given the above mentioned factors, it is not surprising
that fiberglass blisters appear on a large number of vessels which are
kept afloat for long periods of time in relatively warm water.
blisters form only when water penetrates to the laminate. This water
not only damages the laminate by forming blisters causing localized
delamination but also combines with uncured water soluble and hydroscopic
components in the resin forming an acid solution which is highly corrosive
to even the well cured polymers in the resin. As more water reaches
the laminate, more corrosive solution is formed and more resin broken
down. The effect is that of flushing the resin out from between the
fiberglass strands. A laminate so affected is often referred to as having
laminate gets hydrolyzed, a loss of strength takes place mostly in the
form of increased flexibility. The hydrolysis is very layer specific
and does not normally extend to the same degree through all layers.
Consequently, the loss of strength can be quite high in the outer layer
but the overall loss of strength in the hull itself minimal at least
early on in the process.
blistering is a fairly rapid destruction of the outer layer of laminate
(occasionally two or more layers) in the form of delamination due to
the blister action. Typical blistering starts when the vessel is 5-10
years old (applying a barrier coating may delay the formation of blisters).
All blisters will eventually break (unless the entire affected layer
delaminates, a very rare occurrence) given the initial impression of
a blister free bottom. Occasionally, blisters may then again form, this
time further into the bottom laminate creating new and deeper blisters.
(resin depletion) is a rather slow deterioration of the laminate (30-35
years) usually starting in the outer laminates and given enough time
eventually affecting all laminates in the layup. Broken fiberglass blisters
allow the water to reach the deeper layers of laminate more quickly,
but hydrolysis often prevents blisters from forming in deeper layers
due to the porosity caused by the resin depletion (in order for blisters
to form, a membrane capable of holding pressure must exist).
can be recognized by resin "wash out" between the fiberglass
strands leaving a laminate that appears to be "dry", lacking
resin. The resin remaining will be soft giving low barcol readings (below
40-45) and if the condition is severe, the bottom laminate may be easily
deflected by hand pressure and will sound dull when percussion tested.
above, it follows that the most effective way to avoid blisters and
laminate damage is to not expose the laminate to water. This can be
accomplished by keeping the boat out of the water or if you want to
use your boat, by applying barriers to slow down water absorption. If
the boat is new and has never been in the water, a barrier coating can
easily be applied. If the boat is used and maybe even has blisters,
things become more complicated. Please note that hauling a vessel for
winter storage to "dry out" will not significantly, if at
all, lower the trapped moisture in the laminate unless all gelcoat has
of the fiberglass blister repair is to preserve or improve the current
structural integrity and to prevent any future significant structural
weakening of vessel's bottom laminate due to the fiberglass blistering.
The objective must also be to balance the life expectancy of the repair
and the boat against the cost. It is believed that with the materials
and techniques available today, a good barrier coating, under normal
circumstances, should last four to eight years and a proper performed
relamination may last for the life of the boat.
a serious consideration for most boat owners and in most cases spending
money below the waterline instead of above the waterline on cosmetics
and equipment is not a very attractive proposition except possibly for
a "racing bottom". Barrier coating is a way of preserving
"status quo" and preventing additional damage for a limited
period of time, a "temporary repair". Relamination is a repair
of existing damage and a way of restoring or maybe even improving original
structural integrity. With the right materials used, this is considered
a "permanent repair". Relamination costs approximately 1.5
to 2 times as much as barrier coating and is becoming as a very attractive
alternative to the old way of repairing with a barrier coating. This
is particularly true when considering that relamination also addresses
hydrolysis and that the repair often can be delayed for the normal life
span of 1 to 2 barrier coat repairs without significantly affecting
the vessels structural integrity.
below the waterline can be divided into three groups: "paint blisters",
blisters in earlier applied protective coating or antifouling paint;
"gelcoat blisters", blistering of gelcoat with no involvement
of laminate; and "fiberglass blisters", blisters involving
one or more layers of laminate.
can be considered purely cosmetic and should be treated as such. Blistering
of earlier applied protective coating would indicate that the coating
is compromised and would need to be renewed for best possible protection
of the laminate keeping the objective of the fiberglass blister repair
in mind. Coating blisters are 1/16 inch to 1 inch in diameter, most
common are 1/16 inch to 1/8 inch in diameter. Blisters can be punctured
by light finger pressure and contain a sticky acid fluid (always wear
eye protection when opening any kind of blister).
blisters with no involvement of laminate is rarely seen but when seen
is most often found to be one layer of gelcoat blistering off another
underlying layer of gelcoat. This type of blistering is mostly cosmetic,
but can possibly increase the likelihood of fiberglass blisters developing.
Gelcoat blisters are 1/16 inch to 1 inch in diameter, most common are
blisters 1/8 inch to 1/4 inch. Blisters can be easily punctured with
a knife tip. Blisters are "crunchy" and contain a sticky acid
blisters involving gelcoat and part of first layer of laminate is by
far the most commonly observed type of blistering. This type of blistering
causes relatively little structural damage initially and repairs can
often safely be postponed for several years. However, a wet laminate
is substantially less strong than a dry laminate and moisture penetration
into the laminate can remove resin between fiberglass strands, accelerating
the weakening of the structure over time. Blisters are 1/8 inch to 1
1/2 inch in diameter, most common are blisters 1/8 inch to 1/4 inch
in diameter. Some pressure is needed with a knife tip to puncture the
blister. Blisters are "crunchy" and contain a sticky acid
blisters involving one or more layers of laminate leads to rather rapid
loss of the structural strength of a vessel's laminate depending on
number of blisters and layers of laminate involved. This type of blistering
normally calls for corrective action within a relative short time span
(1 to 3 years). Blisters are 1/2 inch to 3 inches in diameter, most
common are blisters 1/2 to 2 inches in diameter. Blisters can be cut
using a knife applying some force. Blisters contain a sticky acid fluid.
a repair is undertaken, the laminate should be inspected to determine
to what degree the laminate is damaged by blistering and hydrolysis.
A "window" should be created in the bottom laminate. This
is done by grinding into the laminate layer by layer until good laminate
is found. Each layer is measured for moisture, hardness (barcol) acidity
and thickness. Layers are also visually inspected for resin and blister
damage. Thickness of damaged layers can be expressed as a percentage
of overall skin thickness. When the information has been analyzed, a
decision can be made as to whether any repairs are needed at the present
time. If a repair is deemed necessary, a decision has to be made as
to barrier coating or relamination taking into consideration the use
of the vessel and expected term of ownership verses cost.
blister repair commonly consists of an applied barrier coating to prevent
the water from entering the vessel's bottom laminate. None of the presently
used barrier coats are 100% water tight and water can, molecule by molecule,
penetrate through the created membrane into the vessel's laminate. Microscopic
cracking of the barrier coat may also occur due to lack of reinforcement
from fiberglass strands again causing water to reach the laminate. Consequently
a fiberglass blister repair using barrier coating is not a permanent
repair but needs to be renewed after a period of time.
experience and observations, the following basic steps may be taken
to assure a satisfactory fiberglass blister repair with maximum life
expectancy. The below outlined procedure is for illustration purposes
only and may vary between repair facilities.
gelcoat should be removed from below the waterline. This prevents
blistering from reoccurring in the interface between the gelcoat and
the first layer of laminate, by far the most common area where blisters
are seen. The removal of gelcoat also promotes drying of the laminate.
vessel's bottom laminate should then be dried to an even moisture
content, comparable to that of the vessel's topsides. On a Sovereign
Moisture Master Meter, moisture content should not be more than 5%
on the A scale (this is not the actual moisture percentage in the
laminate, readings are for comparative purposes only). The reason
for drying is two fold. The less moisture that is trapped in the laminate,
the more water molecules are needed to pass through the barrier coating
before enough pressure builds up to form new blisters. If the laminate
moisture content is too high, many of the barrier coatings used will
not cure or adhere properly. This often results in blistering of the
barrier coating itself or total failure of the bonding of the barrier
the laminate after drying to remove any solids and contaminates that
might have reached the surface of the laminate after the drying process.
This washing can be plain water or an alkaline solution. If an alkaline
solution is used, Ph testing should be performed after washing to
make sure the alkaline solution has been completely removed. Laminate
should be dry before proceeding.
entire below waterline surface should be sealed with one to two coats
of thin resin. This resin should penetrate between any loose fibers
and into any pores left in the surface of the laminate from removal
of the surface, helping to fill and seal as many voids as possible
and to create a good bond for the products subsequently applied. The
bottom should then be faired with a compound containing non-water
fairing is completed, barrier coating should be applied to the bottom
following the product manufacturer's instructions; however, no less
than three coats (not counting coats applied before fairing) should
be applied whether the coating is brushed, sprayed, or rolled. Anything
less than three coatings will not guarantee complete mechanical coverage.
To insure sufficient coat thickness throughout the vessel's bottom,
it is suggested that not less than five coats be applied with more
being better. If sanding is used or called for between coats, the
number of coats may have to be increased.
radical and in general much longer lasting approach to repairing fiberglass
blisters is to remove all damaged bottom laminate, then laying up new
layers of laminate using epoxy or vinylester resin (laminate made with
both of these resins have been shown to be much less likely to blister
than polyester resin). This repair method has now been proven for a
sufficiently long time to almost be labeled as a permanent repair. A
theoretical possibility exist of blisters eventually forming in the
interface between the different resin types or in the old laminate.
Relamination of vessel's bottom is in general only suggested by Hartoft
Marine Survey, Ltd. when fiberglass blistering is seen involving more
than one layer of laminate or when severe hydrolysis has taken place.
for relamination follows that of barrier coating closely except for
laying up of laminates as needed between step 4 and 5, however then
the fairing step is often postponed until after the lamination is completed.
decision has been made to repair a vessel's bottom a number of additional
considerations are needed. The approach to the repair as suggested by
the chosen repair facility should be checked against the above outlined
procedure and a resin/coating system should be selected. It is strongly
recommended that the resin/coating system to be used is one with which
the repair facility is familiar and has a positive experience in applying.
A large number of barrier coating systems from different manufactures
are available today; however, reliable records of effectiveness of the
repairs made in the field with any of the systems are poor, making specific
of the resin systems are well documented and supported by their manufactures.
Not all of the available products are compatible. It is strongly suggested
that products from one system not be mixed with products from another
system unless thorough testing is first performed. Of all the systems
available, the epoxy resin systems appears in theory to be the most
desirable. It should be pointed out that epoxy systems require very
controlled environments and rather precise techniques during application,
which might turn out to be a very limiting factor when used in the normal
boat yard environment. Vinylester resins have been gaining popularity
as a barrier coating and as a resin for use in relaminating, due to
their ease of working and their much reduced sensitivity to environmental
factors, they appear to work very well in the boat yard environment
but under laboratory conditions they do not show quite the outstanding
characteristics of the epoxy resins. The failure rate of vinylester
resins is at present by far the smallest of the commonly used materials
making their use very attractive.
of the system used it is suggested that to achieve the best possible
curing and to lessen the chance of contamination, a controlled environment
should be created around the vessel when coatings are applied. This
can be in the form of a plastic tent with temperature and humidity control
or the vessel can be placed inside a controlled building. Again, it
should be pointed out that a repair facility should be chosen which
is familiar and experienced with the desired coating product and procedure.
to be no advantage to treating fiberglass blisters on an individual
bases except possible pressure relief by puncturing blisters. Any other
approach can at best be considered cosmetic only. When fiberglass blisters
are observed on a vessel's bottom, a determination should be made as
to the severity of the blistering. If only a few scattered blisters
are seen in the bottom involving gelcoat and portions of first layer
of laminate it is suggested that no immediate corrective action be taken
but that the blister condition be monitored at subsequent haul outs
by the "window" process.
blisters are observed involving two or more layers of laminate or severe
hydrolysis has taken place, it is suggested that the overall structural
condition of the vessel be evaluated and balanced against the use (i.e.
"inshore" or "offshore") and corrective measures
be taken within a reasonable time period as outlined above.
1995 HARTOFT MARINE SURVEY, LTD.