Failed Blister Repairs
A Case History and Solutions
by David Pascoe, Marine Surveyor
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 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 successful 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 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.
Multiple causes of blister reappearance
After removal of the newly applied barrier coatings, this is what Mr. Williams hull looked like. 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 old blisters.
Multiple causes for the reappearance of the blisters became immediately apparent. These are as follows:
Failure to correct imperfections
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 them.
Fairing material that either turned soft or was not properly mixed and catalyzed.
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. If the voids are not removed, then the potential for reformation remains.
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 polyester. Otherwise, it's 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 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 it's certainly reasonable to assume that
Scrape marks were made with the end of a ball point pen to indicate how soft it is , a perfect environment for blister formation. This is the point where the bottom was faired into the old gelcoat at the boot stripe. Note blister voids at lower center.
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.
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.
Myth about blisters
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 recurrence. 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 poorly 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 further on.
Causes of blisters and factors involved
While there is a great deal of myth about blisters, we have learned so far that:
There are a number of factors involved in why this is so:
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.
Bottom line to blister repairs
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.
First posted on Sept 2, 1997 at David Pascoe's site: