Banishing Bottom Blisters
If your classic-plastic cruiser contracts the pox, take these steps to ensure years more of trouble-free service. "Projects" from our April 2012 issue.
After 20 years and many thousands of miles, our 1961 Pearson Invicta developed some hull blisters. After fixing those, the hull remained solid as a rock until we sold the boat in 2010. We now know that a reasonably well-built glass boat will last more than five decades. We also have learned to expect some laminate deterioration in the outer layers of the hull below the waterline when dealing with “mature” vessels. When my wife, Nancy, and I bought a 1990 Mason 44, a thorough examination of the hull was the top item on the long work list we drew up to prepare the boat for high-latitude sailing. The first step was to blast off the thick layers of old antifouling paint. Once stripped, the hull sported several obvious bumps in the gelcoat. Opening them with a grinder revealed patches of dry laminate, but luckily, no foul-smelling liquid seeped out.
Many below-the-waterline issues on older fiberglass production boats can be traced to their original layup. First, the gelcoat was sprayed into the mold to ensure a gleaming hull. To guarantee an absolutely smooth surface, the builders next laid down a layer of random mat, which unfortunately was difficult to saturate evenly with resin. The builders could’ve employed better materials, like glass cloth or woven rovings, but those cause an unsightly “print out” in the finished hull. That’s why many owners of classic fiberglass boats end up with a hull consisting of a hidden layer of mat that’s full of dry spots unsaturated with resin. Over time, the aging gelcoat becomes permeable to water, and the unsaturated spaces in the mat may start blistering. With luck, this results only in small areas of dry glass. However, if the laminators were sloppy mixing the resin and hardener or the quality of resin was poor, the boat will suffer from the infamous weeping pox.
Weeping pox. For more photos of Tom's bottom job, click here.
Essentially, dealing with a deteriorated glass hull boils down to shaving off the rotten material—by blasting, machine peeling, or grinding—until the healthy, solid laminate is exposed. If it’s dry, coating with several layers of an epoxy-based barrier coat can prevent future problems—but remember: This job is harder than it sounds. Even then, you must determine the moisture content of the remaining glass; most boatyards have experience working with moisture meters or will call in someone who has. The hull must be completely dry before applying epoxy barrier coats. Leaving the boat on the hard will do it, even though, in the worst cases, this may take months, or the hull can be tented with dehumidifiers.
The accompanying photos illustrate how we addressed the blistering issues we discovered on our Mason 44. If you find, after peeling the gelcoat, that a boat has a lot of blisters oozing smelly liquid, these should be washed repeatedly with high-pressure water. All blisters should be ground down to the solid glass layers. Once the hull dries, fill the small blisters with epoxy resin thickened with colloidal silica. Blisters measuring more than 2 inches in diameter should be rebuilt with fresh fiberglass.