John Williams is the proud owner of a ten
year old 35' sloop. Prior to his purchase, he spent nearly a year
searching around the country to find this particular yacht because
it ideally suited his needs. When the survey was conducted, there
was only one significant problem with it: it had a scattering of
small blisters on the bottom, which I usually refer to as "pimple
rash" to differentiate this condition from considerably larger
blisters. The blisters in this case were no larger that 1/4"
in diameter and had a density of about 2-3 blisters per square foot
if averaged over the entire bottom area.
John lived in California and eventually moved to
boat from these cool waters to Florida. Suddenly the 90o
waters of Florida's waterways caused the number of blisters the
number of blisters to blossom from perhaps a few hundred to several
thousand. Not liking what he saw, he decided to have them repaired.
Obtaining three estimates on the cost, he finally settled on the
Ace Boatyard, in part because they used the West Epoxy system and
Williams had heard that this material was highly successfull at
solving the blistering problem. The cost was $7,000 and he was given
a 5 year guarantee. He was also told that the repair would eliminate
his blistering problem, although the yard manager did tell him that
it was possible that "a few" blisters could possibly reappear.
The repair method included stripping off all the
paint and old gelcoat with a specially designed machine by an outside
contractor. This was followed by "fairing" and recoating
the bottom according to the instructions provided by the manufacturers
of the West System. In addition, it also included "hot coating"
the bottom, a method described to me as as applying the antifouling
bottom paint to the bottom while the last coat of West System epoxy
was still wet.
The job was completed, Mr. Williams paid his bill
and went on his way, pleased that his blistering problem had now
been repaired and solved. At least until a year later when the yacht
was hauled and he discovered that about 50% of the blisters had
reappeared. Returning to the yard that did the work, his complaint
was greeted with a response somewhat different than what he was
told prior to giving the yard his $7,000. Now the blister job was
no longer a cure for the problem but simply a repair of the existing
blisters. Moreover, all of the blisters that reappeared were new
ones they said, unrelated to the ones just recently repaired. That
meant that, although Ace Boatyard did indeed warrant that the blisters
they repaired would stay fixed, the new blisters were not a reappearance
of the old blisters, and therefore not covered by their 5 year warranty.
Mr. Williams estimated that about 1/3rd of all
the blisters returned within one year, and he wasn't buying Ace's
revision of their warranty. He was told that the repair would end
his blistering problem, but it did not. The yard showed no sign
of wanting to compromise the matter so he sued.
The yard's defense counsel hired a surveyor to
look at the boat, and after doing so pronounced that the entire
hull had severe delamination problems, determined by "sounding
with a phenolic hammer." Nothing else was done to verify the
"delamination." The yard then hung their defense on the
premise of preexisting manufacturing defects as the reason why the
repair wasn't successful and blisters recurred.
Called as experts for Mr. Williams, we examinated
the yacht after the newly applied bottom coating had again been
removed, the bottom being stripped down to the skin out mat and
in some cases right down to roving. Our sounding of the hull produced
not the slightest indication of even possible delamination of the
of the newly applied barrier coatings, this is what Mr.
Williams hull looked like. Knife blade is inserted into
the void spot caused by the old blister. Most, if not all,
of the original blister voids remained. At right, the new
resin can be seen to have been applied directly over the
Note: The term "mat" or "skin
out mat" refers to a fiberglass fabric made up of chopped
fiberglass fibers that are quite short, usually about 3-4"
long. These fibers are oriented in all directions and are not
interwoven or interlocking, which is what mades the material relatively
weak compared to woven fabrics. Mat is laid against the gel coat
that is sprayed into the mold precisely because it does not have
a weave pattern which would telegraph through the gel coat to
give the hull finish the same texture as the fabric. The downside
of its use is that it is very difficult for the laminators to
make sure that the material is fully impregnated with plastic
Multiple causes for the reappearance of the blisters
became immediately apparent. These are as follows:
- As shown in nearby photos most, if not all, of the old blister
cavities were still present. As far as we could determine,
no effort was made to grind away the cavities or void areas
and fill them.
- Scattered and random areas of the original skin out mat,
as part of the original lay up, had significant areas of unsaturated
fibers and minute voids, i.e. air bubbles in the original
- Some areas which had been faired with an unknown filler,
but looks like two-part epoxy, were very soft and pliable,
giving the appearance that it had not been mixed in proper
ratios and did not cure properly. Since some of this material
was hard, and some soft, we do not consider it likely that
the material softened of its own accord.
- Judging by the coloration, it was apparent that two applications
of a clear resin (although a few areas showed three) and one
layer of a fairing material had been applied, the former by
roller without being brushed out. In most areas where the
barrier coating remained, it was usually found to be very
thin, notably thinner than a typical gel coating. We estimate
this at about 10 mils. A thick gel coat would be 30 mils,
a thin one 20 mils.
The failure to correct these imperfections provided
the basis for the reformation of both the old and new blisters.
Bearing in mind that the movement of the yacht from cool waters
to the 90o waters of the canals of Ft. Lauderdale resulted
in a very rapid development of blisters, the yard had every reason
to believe that the blistering of this hull was likely to continue
at a rapid rate since Mr. Williams had explained all of this to
|Although these blisters were ground away, the
void spots or air bubbles within the skin out mat that initiated
the blisters are clearly evident in this photo. The circled
insets are enlarged for clarity. The polyester plastic here
is not hydrolyzed (disolved) as some researchers claim is
the cause of the problem. The plastic is hard and unaffected.
If these voids are not removed, then the potential for reformation
remains. Also note how dry some of the fibers are.
Assuming that the moisture meter used to determine
that the hull had indeed dried out prior to recoating was accurate,
this case, and many others like it, seem to belie the common
notion that epoxy resins are significantly less permeable than
polyesther. Otherwise, its difficult to explain why large numbers
of blisters reappeared so rapidly. Either the hull was never
really dry, or somehow it once again absorbed water.
Another shortcoming in the repair process was
found, that being that the new coating had been applied with
a paint roller and never leveled out. This left a surface texture
that was quite rough, resulting in a surface mill thickness
that was very irregular. Whether this had an affect on the reblistering
hasn't been empirically determined, but if barrier coat thickness
has anything to do with the rate of permeability, then its certainly
reasonable to assume that it did.
|Fairing material that either turned soft or
was not properly mixed and catalized. Scrape marks were
made with the end of a ball point pen to indicate how
soft is is, a perfect environment for blister formation.
This is the poinit where the bottom was faired into the
old gelcoat at the boot stripe. Note blister voids at
It also appears that a two-part epoxy filler
was used in the fairing process that amounted to no more than
25% of the bottom area. This included spot filling depressions
of what might have been larger blisters, as well as fairing
around through hull fittings, fairing the waterline into the
removed bottom gelcoat, as well as just general fairing. The
fairing process was poorly accomplished, resulting in a "lumpy"
appearance to the finished job. Most significantly, as we have
found in a large number of other failed blister jobs, the fairing
material, whatever it is, was found to be soft and pliable.
Whether this resulted from improper mixing and incomplete curing,
of if the material softened from water exposure or resultant
chemical reactions, was not precisely determined. However, it
would certainly be difficult to sand a filler that was not completely
cured, and the fact of the lumpy appearance of the bottom suggests
this possibility. On the other hand, our finding of so many
other fillers that turn soft also suggests the likelihood of
problems of (1) incompatibility of materials being combined,
or (2) that these materials are severely affected by contact
with sea water. All we can say for certain is that it is a two-part
mix and that the material in many areas, although not all, was
A final flaw was discovered in that many of
the blisters (we don't know what percentage because 90% of the
bottom paint was removed) had occurred not under the new epoxy
coating, but between the new coating and the new application
of bottom paint. In other words, they were bottom paint blisters.
And paint blisters are often misinterpreted as barrier coat
Since there was only one type and coating of
bottom paint on the hull, this can't be attributed to incompatibility
of bottom paints which, as we know, is usually the cause of
paint blisters. So why would a brand new antifouling coat blister
like this? Did it have something to do with the "hotcoating?"
We would have to conclude that it must have, for there would
seem to be no other explanation. Examination under magnification
suggested a possible answer, since each blister that we dissected
and examined revealed a pit, or indentation extending into the
new epoxy barrier coat. The presence of the pit would suggest
that a chemical reaction did indeed take place to cause the
erosion that created the small pits and blisters in the paint.
Obviously, this begs the question of whether
applying an antifouling coat to a wet epoxy barrier coat is
a good idea. It may save the boat yard the task of sanding the
bottom before painting, but it certainly didn't make the paint
adhere any better.
|Comparison view of fully saturated skin out
mat at left, poorly saturated mat at right also showing
numerous void spots or air bubbles. Notice that no blisters
appear in the fully saturated laminate. These two areas
are the same boat hull. Doesn't this tell the real story
of how and why blistering occurs? If the laminate is fully
saturated, blisters CAN'T develop.
As we have stated in other articles, blister
repair failures like these are becoming commonplace. One reason
is due to the fact that there is such a great deal of misinformation
floating around out there about the nature of the problem. Over
the last decade, I have looked at thousands of blistered boat
bottoms and I know one thing for certain. I have never seen
blisters occurring in a laminate that did not have voids or
unsaturated fibers. It can't happen because there must first
be a void space of sufficient size to collect water in sufficient
volume to initiate the blistering process. I am convinced that,
lacking the voids, blisters cannot form.
The significance of this is twofold. First,
for builders it means that if one ensures that there is a thorough
wet out of the skin out mat, resulting in fully saturated fibers
and minimal voids between gel coat and mat, and between mat
and first structural layers, blisters will not form unless you
are using third rate materials.
|This photo, not this essay's subject vessel,
shows a boat bottom with at least two dozen grind spots
in little more than one square foot area. Despite all
the grinding, hundreds of voids and areas of unsaturated
fibers remain. When the condition of the skin out mat
is this bad, it cannot be successfully repaired. It must
be removed completely. Unfortunately for the owner, the
yard simply filled the holes and recoated it, with a high
probability that the repair will fail.
Secondly, the same point applies to repairs.
The uncorrected problems associated with Mr. William's failed
blister repair are common to nearly all others. If the repairer
eliminates the voids that help initiate the problem in the first
place, he eliminates most of the potential for recurrance. Most
repairers are knowledgeable enough to know that they have to
remove the existing blister voids, and do so. Yet from touring
boat yards and watching their process, it is clear that most
are not dealing with the problem of poorly saturated fibers
from the original construction. Recoating over a pooly saturated
skin out mat occurs again and again.
This presents the repairer with something of
a problem because to eliminate the unsaturated fibers in the
skin out would mean that a lot of it has to be stripped away
and replaced. The problem here is that this would significantly
increase the cost to the customer that is likely to meet with
resistance. There is a solution to this that will be explained
While there is a great deal of myth about blisters,
we have learned so far that:
- The vast majority of blisters occur between the skin out
and the gel coat.
- A slightly smaller percentage occur within the skin out
mat, or between the skin out and the first structural laminate.
- Blisters occurring within the structural laminate, particularly
woven fibers, are extremely rare.
- Hydrolysis, or dissolution of the plastic, is not an initiator,
but a secondary reaction of water in the laminate. Softening
of the plastic does not usually occur until blistering is
well advanced, and often doesn't occur at all.
There are a number of factors involved in why
this is so:
1. Chopped strand mat is difficult to fully
saturate and a very high percentage of all boats have unsaturated
mat. That some do not blister suggests the use of far less permeable
gelcoats and resins, or that some resins are more chemically
stable than others.
2. The formation of blisters is associated
with the softening of the barrier coat due to the presence of
precipitated solvents from the resin (styrene) in a void space,
and the build up of slight gas pressure sufficient to form the
blister. But the pressure build up has been found to be very
weak, so that the deformation of the blister may be less due
to pressure than the effect of expansion of the gel coat caused
by solvent softening. The mechanism is much the same as pouring
acetone on cured paint, causing it to wrinkle.
3. Blisters do not form in the structural laminates
if only because these heavy fibers are too strong to permit
deformation of the laminate. On the other hand, gel coats and
barrier coats are not fiber reinforced and are thus far more
prone to surface deformation. For a blister to develop from
a tiny air bubble between layers of roving, for example, would
require a very high pressure indeed to result in deformation.
On the other hand, random directional mat fibers are quite weak.
4. These points lead to the seemingly inescapable
conclusion that poor saturation of skin out mat has to play
a pivotal role in blister formation. If anyone has any doubts
about this, try taking a tour through the boat yards and see
if you can find blisters occurring within thoroughly saturated
laminates. I can tell you in advance that you won't.
It is reasonable to conclude from this that,
while epoxy and vinylesther resins are still sufficiently permeable
to fail to prevent blistering, the elimination of most of the
unsaturated fibers in the skin out mat will preclude most of
their reformation when combined with higher quality resins.
It is foolish for a boat yard to give a 100%
guarantee on blister repairs, for it is not possible to eliminate
all voids close to the surface. Yet it is possible to remove
90% of the voids through careful preparation. Bear in mind that
if it requires completely stripping the skin out mat, then there's
no reason not to do so. It doesn't have to be put back on since
the only reason for it's presence is to prevent telegraphing
of roving pattern to the gelcoat, and that's not a consideration
on the bottom of a hull. If the structural layers prove to be
well-saturated, as they usually are, then the problem is going
to be 90% solved.
||Here is a particularly good shot of unsaturated
fibers in a skin out mat. These really stand out because
the layup resin in this hull is tinted dark blue. In most
cases, the poorly saturated fibers do not stand out this
well. In this case, as in so many others, the repairer
simply applied a new barrier coat on top of this mess
and the blisters reappeared with six months.
The bottom line to blister repairs is that
there are far too many people in the business who don't know
what they're doing. They apparently are not aware that for the
repair to be successful, they must eliminate the defects that
caused the blisters in the first place. Yet it is not possible
to determine all of the factors that cause blistering, especially
the cause of water getting into the laminate. The buzzword is
"osmosis," as if permeability of coatings is the only
means of water saturation. The reality is that we can identify
a half-dozen ways that water can get into a laminate that have
nothing to do with exterior coatings. So even if there were
a totally effective, non-permeable coating, it would not solve
the problem, for you can't prevent the absorption of water from
the interior of the hull, or around through hull fittings and
But the one method that offers the greatest
possibility of a cure is to eliminate the voids within the outer
laminations where blisters commonly form. And if that means
stripping the chopped strand mat from the hull, then that is
what has to be done. Otherwise, its just money down the drain.