Fiberglass Hull Gel Coat Blistering
This is s subject that come up from time to time so I thought that I’d share some of my views on it here.
Hull gel coat blistering is an issue that has been around for about thirty years or so and to this day is continues to plague boat owners, manufacturers, brokers, surveyors and anyone else involved with fiberglass boats.
One would think that after this much time a solution to the problem would have been found but that does not seem to be the case and the condition continues to confound the marine industry almost daily.
The above picture is a good example of gel coat blistering. These blisters when probed excreted a small amount of clear water. This indicates that they are likely limited to the gel coat only and do not effect the laminate layers. If that were the case the liquid excreted would have been milky in appearence and with a vinager odor.
Gel coat blisters are the result of air pockets or voids that sometimes exist between the hull’s exterior gel coat layer and the layers of fiberglass mat that make up what is referred to as the skin out coat that are used in a hull’s exterior laminate layers.
When a fiberglass hull is constructed the builders start with a clean female mold. Coats of gel coat are sprayed on and allowed to set until they cure to a “tacky” state. Next thick layers of chopped strand mat are applied to the still sticky gel coat and then are saturated with resin. This process is usually performed by hand using simple rollers of various configurations. This thick layer of mat is used to help prevent the “weave” pattern of the woven fiberglass material applied behind the mat from appearing on the surface of the hull’s exterior gel coat surface. The nature of this construction method is that the workers are continually working from the blind or back side of the hull’s laminate layers. When applying the thick layers of mat it is very difficult to ensure full saturation with resin and dry areas or voids sometimes occur. These voids in the laminate fill with water and manifest themselves into blisters.
The next question that needs to be answered is how does this water find its way into the void area? Gel coat porosity and hygroscopic action are the most likely causes. This is also where the “osmosis” word usually comes into play and usually incorrectly I might add. We’ve discussed the “O” word in previous posts so I won’t get into it here. Basic chemistry teaches us that water, being the smallest and most basic of all molecules can pass through the larger and more complex gel coat molecules. Then through capillary action this moisture travels along the short strands of spun glass that make up the mat layers, eventually finding their way to the void areas that were built into the hull laminate and blisters are formed. Different gel coat formulations appear to resist this better than others.
At this point a couple of conclusions can be drawn:
Gel coat blistering without is unlikely to occur without existing voids in the hull laminate.
- A hulls likelihood of developing blisters was largely determined on the day that it was manufactured both by the quality of the manufacturing processes and the materials used.
- Just because a hull shows elevated moisture levels when tested does not mean that it will necessarily develop blisters.
The process described above is the most common cause of hull blistering but may not be the only one. I’ve seen small hard, dry blisters that appear to be confined to the gel coat itself. After discussions with numerous fiberglass repair people and surveyors these may be caused, by a chemical reaction between the gel coat formulation and some other substance or possible by improper catalyzation of the gel coat. Blistering can also occur between anti fouling paints and the hull surface and with barrier coats as well.
One important observation that I’ve made over the years is that hulls manufactured prior to about 1980 rarely develop blisters. My theory for this is that in those days fiberglass manufacturing was largely in the experimental stage and most boat builders had a very limited understanding of the materials used and the processes involved. They simply overbuilt everything and used the best quality materials available. This has resulted in many hulls that have withstood the test of time quite well. But before you run out and start shopping for a pre-1980 boat another thing that I would like to point out is that on these older boats the decks and cabin superstructures have not fared nearly as well. The use of balsawood as a core material became common and with the gel coat degradation has occurred over the years largely due to UV exposure on the horizontal surfaces. This has resulted in moisture penetration of the core material and “soft decks” have been the result.
The next question is, does blistering have a negative impact on the structural integrity of a fiberglass hull. The answer, in most cases is no. Blisters are very unsightly and can affect the sale ability of a particular boat but unless the condition is extremely bad it rarely causes structural issues. This is not to say that it can’t but here in the great white north where boats spend six months of the year out of the water which allow the hulls to dry out and considering the fact that fresh water usage does not promote osmosis and blistering in the way that salt water does the condition rarely becomes a structural issue. It’s just plain ugly.
Another fact that needs to be pointed out that in recent years newer manufacturing processes such as vacuum bagging, resin infusion and the use of higher quality materials such as vinyl ester and epoxy resins have reduced the tendency of hulls to develop gel coat blisters to a large degree. It does however still appear that the condition has not been completely resolved and it continues to plague the marine industry to this day.