See Pictures of the re-lamination
project.
We had of course heard of osmotic blistering, and were not really interested in buying a boat with this rather severe problem. A Norsea for a low price and assurances that the blistering was not all that severe got us into the mess. As soon as I began removing material I saw that the damage to the hull was extremely extensive, and that just patches were not going to be sufficient.
The Osmotic Blistering Process
Differences in Materials
Repair Strategies
Eva's Quickie Relamination
The Osmotic Blistering Process
There has been much debate about what it is excactly about polyester resin that makes it prone to osmotic blistering, but in the end it is the simple fact that polyester reacts slowly with water that causes the problem. After many decades of soaking in the water the polyester becomes saturated with water and eventually pockets of liquid may develop around impurities in the lamination. Once a very small pocket of liquid develops this liquid rapidly becomes thick and acidic from the breakdown of the polyester resin. The low pH of the pocket of liquid contributes to further breakdown of the polyester resin, and the high ionic concentration in the liquid sets up an osmotic pressure gradient where the new blister begins to strongly pull more water in through the polyester. As more and more water is pulled in the blister grows and forces the layers of lamination apart. Generally blistering starts in the outer layers of a hull, and then works it's way in farther as the hull becomes more and more saturated with water.
Differences in Materials
Polyester hulls often suffer from osmotic blistering, but epoxy hulls do not ever suffer from osmotic blistering. Most fiber reinforced plastic hulls from the 1970's through the 1990's were made out of polyester because it was so much cheaper, and from some perspectives easier to work with than epoxy. There are however several types of polyester resin, with somewhat different structural and chemical properties. The cheapest and most common type of polyester resin that was available for quite some time has become known as orthophthalic polyester resin and is said to be the most prone to osmotic blistering. The other type of polyester resin is known as isophthalic polyester resin, and is not only slightly stronger but also somewhat more chemically resistant and therefore less prone to osmotic blistering. After osmotic blistering of hulls became a significant problem in the 1980's many boat builders switched to using the somewhat more expensive isophthalic polyester resin. Another similar resin called vinyl ester has also become very popular because it is significantly stronger than polyester and extremely resistant to osmotic blistering. In recent years many high end boat builders have taken to using a substantial layer of vinyl ester lamination on the outside of the hull, while still relying on the much cheaper polyester resin for most of the structural strength of the hull. Epoxy is also essentially fully resistant to osmotic blistering, and many hulls from the 1960's were laid up entirely with epoxy. In recent years there has been a resurgence in the use of epoxy for boat hulls, and most high end boats are built entirely out of epoxy.
Repair Strategies
The basic strategy for repairing osmotic blistering is to dry the hull and then seal it with epoxy to prevent it from hydrating again. Epoxy is not only impervious to blistering since it is much more resistant to breaking down, but it also allows only a much smaller amount of water to pass through it. A good blister repair for most boats that have just begun to blister is to store it out of the water for some period of months or years until the hull is once again dry as a bone. If this is done every few years then it is not even necessary to put any epoxy on the hull. Just the factory installed gel coat was sufficient to protect polyester hulls from rapid hydration, and many isophthalic polyester hulls have somehow managed to survive decades of use. If a boat is to be left in the water all the time then osmotic blistering becomes much more likely, and once osmotic blistering has begun repair is much more difficult if long periods of disuse are not permissible. The basic strategy is however exactly the same, the hull must be dried and the water must be prevented from coming back. The fast and somewhat effective solution that was developed was to dry the hull out for a shorter period of a few weeks and then apply an epoxy barrier coat to keep the moisture from coming back. The amount of time out of the water required to get a hull significantly dryer depended on the temperature and humidity of the area. Hulls dried much quicker in the California sunshine than in the high humidity and frequent rain showers of Florida. The farther the hull was moved back from the water front the faster it dried. A system of electric heated pads that could draw a vacuum on the hull became popular for reducing the time required to remove a significant amount of moisture. In the end though it was sealing the hull against further hydration that was necessary to stop osmotic blistering. Many coats of epoxy resin painted on up to a thickness of a thirtysecond of an inch did wonders for keeping the water out, and became the standard approach for fixing blistered hulls. The problem with this standard blister repair is that short drying times don't get the hull very dry, and the thin layer of epoxy still lets water through. These standard blister jobs tended to be good for only a few years, then the whole job needed to be done again. The problem with the painted on epoxy barrier coat is not only that it is so thin that water gets through it, but also that it is easily damaged and sometimes develops cracks that allow water through. The only real solution is a re-lamination. Reinforcing the epoxy not only allows a thicker barrier coat to be added, but also prevents cracking and other types of damage to the protective layer. Hulls re-laminated with an eighth of an inch of new epoxy laminate do not suffer from the osmotic blistering coming back.
Eva's Quickie Relamination
We originally planned to simply let Eva's hull dry for the summer, patch the holes and paint on a few coats of epoxy barrier coat. Since we intended to use her as a trailer boat we figured we would not have much more trouble with blistering. When we saw that the blistering had gone nearly all the way through the hull allong the keel though it was obvious that something more drastic needed to be done. The worst part of the job was getting the old barrier coat and damaged material off of the hull. Once the hull was stripped down to virgin undamaged material we filled in the big holes with layers of matt and epoxy, faired the whole hull with epoxy filler and then laid the re-lamination over the top. Very often hulls are peeled back uniformly by an eighth of an inch so that after the new material is laid on the finished thickness is the same as it originaly was. Since material removal was the worst part of the job we decided we could live with a slightly thicker and heavier hull. The worst blistering was isolated to the keel area, and this is where we put on the very substantial eighth inch laminate of two layers of one and a half ounce per square foot matt. On the rest of the hull the blisters were mostly much smaller, and we re-laminated with just two layers of six ounce per square yard cloth. Over the biggest and deepest blisters on the higher part of the hull I also added an extra two or three layers of six ounce cloth in a single coat to provide a dense and strong reinforcement to the hull. In the end Eva is only about a hundred and fifty pounds heavier with the new epoxy bottom and we have often been extremely thankful for both the blister free bottom as well as the extra strength provided by the re-lamination.
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