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Project: Osmosis Repairs
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Projects:
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| Over time, Daydreamer developed a rash of small, pinhead-sized blisters pretty much all around the hull at the waterline. We didn't know if this was a localized problem or a harbinger of larger osmosis problems to come later, so we elected to do a complete hull repair rather than trying to do a minor "grind and patch" fix. This page is about that full hull repair. When a conventional fiberglass hull is being laid up in a mold, there is a sequence of steps usually followed. First, gel coat is sprayed into the mold, then several layers of fiberglass mat are laminated against the gel coat, then alternating layers of roving (woven or unidirectional) and mat are used to build strength and thickness. The outer layers of mat don't really contribute hugely to the strength of the hull but rather are intended to prevent the roving from "printing through" to the outside of gel coat. Print through happens when the coarse weave of the roving can be seen in the surface finish of the gel coat and is usually most noticeable if the outer surface is of a darker color and has a high gloss finish. Current best practice for repair of osmotic blistering is to first remove the gel coat and outer layers of mat then dry the hull out by either letting it set for an extended period exposed to the air or by force-drying using heat and/or vacuum. Once the hull has dried enough, the layers of mat removed earlier are replaced and the hull refinished. When Daydreamer's hull was being laid up in the mold, it had a slight groove or channel at the design waterline and another groove slightly above it at the boot top included as part of the lay-up. It was nice to have this indication of the waterline as it provided a clear marker for masking tape when painting the bottom. It was nice but also ultimately nasty as having these grooves as part of the lay-up in the mold was, we think, the proximate cause of the pinhead blistering we found. Gel coat doesn't have any real strength, it's more like a thick coat of polyester paint, so the molded-in grooves needed reinforcement. Mat will do a reasonable job of conforming to irregular surfaces but it wouldn't fit into the outer corners of the grooves. It turns out that the yard used a fine-weave fiberglass cloth embedded in a bondo-like material as backup behind the grooves, then they proceeded with the lay-up as per normal. And we now know that bondo will absorb water and cause blistering ... Whoops! In retrospect, we might well have been able to get by with a grind out and patch around the waterline. But 20-20 hindsight wasn't available when we made the decision to proceed and it still seems to us that if something's going to be done, then it should be done properly.
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