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What's the Deal With Blisters?

Updated: Jan 22, 2019

“Hey Krabby, What's a blister?” asked the East Coaster bringing up the drinks from below so as to enjoy the late afternoon air on deck.


“That's what I call someone who shows up when the work is done,” I quickly replied.


“No Krabby, I meant the ones on the bottom of a boat, like the two well-maintained fiberglass boats that were in the yard a few months ago. You had to strip the bottoms down and repair the spots that looked like chicken pox. Why do these boats that otherwise look to be in good condi on have this issue?”




“Ok, since you brought the drinks, I’ll tell you the story about osmotic blisters - the condition you observed on these boats below the waterline. There are different theories about the details of why they occur more readily in some FRP ( fiberglass reinforced plastic) hulls than others but the basic science is best to understand first.


"Production FRP boats are built in a female mold. The gel coat (the smooth shiny outside finish of the hull) is sprayed into the mold first, then the structural layers of fiberglass coated with polyester or vinyl-ester resin are placed into the mold on top of the gel coat. When this cures you have a finished hull that comes out of the mold. The basic issue that starts the blistering process is that the gel coat is not as waterproof as the structural FRP substrate. Over time the water works its way through gel coat even slower than into the FRP, where it reacts chemically with the remaining solvents in the cured resin. The water pressure in between the gel coat and FRP is greater than that of the pressure outside the boat and cannot exit out through the gel coat fast enough. Something’s got to give and the gel coat delaminates from the FRP.


When you pop one of these blisters, it has a liquid inside that smells like vinegar. This is the result of the reaction of the sea water and the resin. The different theories of why this happens vary around the different blends of resins and catalysts and quality control during construction. But, we would be here until the sun comes up if we discuss all of that.


The important thing to know is that blisters can occur in any type of FRP construction that uses a resin that has solvents. In recent years, advancements in the manufacturing process and quality control has reduced frequency of osmotic blisters. Some FRP hulls are built with epoxy resin and avoid this issue because there are no solvents in epoxy resin." I paused, nally taking a long swig of my drink.


"Then, why don't they just build the boats with epoxy resin?” asked the East Coaster.


"Good question. Epoxy is a better alternative because it is a complex polymer that has superior waterproof and strength properties, but it dries slowly and is expensive. In the world of production boat building, it increases the cost beyond what the average consumer is willing to pay.”


“Is the repair process a long term cure?” she asked.


“Well not exactly. To repair the blisters the bottom paint is removed along with the delaminated gel coat in way of the blisters. The surface is then allowed to dry out trapped moisture. The blisters are repaired with an epoxy filler and the entire surface is coated with an epoxy primer that acts as a moisture barrier. The barrier coat keeps the hull from absorbing more moisture, however, the moisture that the hull has already absorbed over time cannot all be removed. This remaining moisture can cause a few problems over time - but in most cases, it is minimal and can be managed during routine haul-outs.” I concluded.


“Ok, that’s a lot of good information, I better fix us a couple more drinks,” she said collecting the empty glasses. “It’s sure nice relaxing on this 90-year old boat, re-built and covered in epoxy.” 

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