Banishing Bottom Blisters

If your classic-plastic cruiser contracts the pox, take these steps to ensure years more of trouble-free service.

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 (see photo above).

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 photos above 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.

The Mason’s hull, either by luck or due to sitting on the hard for several months, was dry. Before we rolled on multiple layers of epoxy barrier coating—which should protect the laminate from water for years—it was a good time to remove the gelcoat and find out what we really had on our hands. A guy with a peeling machine shaved the old gelcoat from the keel to the waterline. Now completely “bare,” the hull showed extensive areas of dry, unsaturated mat strands, and the peeling operation left slight furrows. The boat’s hull under the waterline needed some fairing.

To protect myself from fiberglass particles, I zipped on a white Tyvek suit, put on a respirator and ear protectors, and went at it. Here I made the mistake of grinding with 5-inch discs of 36-grit sandpaper. I should’ve used a grinder capable of carrying 8-inch discs sitting on a soft pad. Better yet, I should’ve hired someone to do it. I did remove all the dry fibers but, due to the small disc, the surface still needed fairing. I troweled on epoxy resin thickened with colloidal silica, both WEST System products. This had to be sanded before rolling on coats of Interlux’s InterProtect 2000, an epoxy-based barrier product. First, I washed the surface with water and light detergent in order to remove the amine blush, a waxy film produced by curing epoxy. A 44-foot boat with a long keel, a 32-foot waterline, and a nearly 7-foot draft towered over Nancy and me like a cliff of cured epoxy as hard as granite. The task of fairing it all with orbital high-speed sanders, followed by wet sanding, took us three long days.


Rolling on the InterProtect 2000 doesn’t require sanding in between; we easily put two coats on in a day. However, the stuff contains some strong, nasty solvents, so we wore respirators and gloves. I followed the sixth coat with two rounds of antifouling paint. All that was left to do was to move the jack stands and block the boat higher to address the untreated spots.

The project, carried out in the Florida summer heat, took some stamina. I also added a bit of extra work for my own piece of mind and satisfaction. Many fiberglass boats, including ours, were built in split molds. After the hull was laminated, the mold had been opened sideways, leaving a visible seam that ran from the stem along the bottom of the keel and up to the sternpost. The builders had filled it with cheap polyester putty and sprayed it with gelcoat. Though it shouldn’t affect the strength of the hull, in our case the peeling process opened up this seam; with all the crumbling putty, it looked awful.

To rectify it, I removed the loose putty first and filled the gaps with thickened epoxy resin. Next, I covered the seam with a layer of biaxial cloth and epoxy resin right along the boat’s backbone, from the stem over the keel to the stern. It took a few more days of extra fairing and sanding, but it made me much happier, especially when I recalled past collisions with tree trunks at sea.


Removing a certain amount of the deteriorated laminate may seem worrisome. However, remember that the gelcoat and the resin-starved mat didn’t contribute anything to the strength of the hull. Early glass boats often had extremely thick hulls, and stripping the outer layers won’t affect the strength. Just because it was built a long time ago by a reputable builder doesn’t guarantee a good laminate job. It basically comes down to this: Who was the foreman of the glass shop? Should you need to dig deeper than the gelcoat and mat to reach the solid laminate, seek professional advice from a truly experienced surveyor or a respected boatyard foreman. Some beautiful yachts with great sailing qualities have had to be reglassed totally within 10 years of launching by owners dedicated to the design. On the other hand, many hulls never contract blisters after decades afloat. If yours isn’t one of those, this project to eradicate hull blisters should keep a yacht going strong for many decades.

Longtime Cruising World contributors Tom and Nancy Zydler have refit their Mason 44, Frances B, for extensive high-latitude voyaging. This article first appeared in the “Projects” section of Cruising World‘s April 2012 issue.

Sailboat Blister

Sailboat Blister

This troubled bottom of weeping blisters will take a long time to dry and get ready for the treatment. Tom Zydler
Boat Blister 2

Boat Blister 2

In this close-up, the thin inadequate coating of the epoxy barrier coating shows as blue gray at the edges of the blister, which exploded from the osmotic pressure within the laminate. Tom Zydler
Boat Blister 3

Boat Blister 3

The previous two pictures showed images from another boat. Aboard our Mason 44, a blasting job meant to remove the layers of the antifouling paint from our boat also uncovered a crop of blisters. Tom Zydler


The peeling cutter removed the gelcoat from the hull under the waterline. This revealed extensive patches of mat that were never properly saturated with resin. Tom Zydler


This photo shows the seam of crumbling putty we discovered along the stem. The black spots show where I began filling the worst voids with thickened epoxy. Tom Zydler
How to Get Rid of Sailboat Blisters 6

Sailboat Blister 6

In this photo, Tom is working hard with the roller to thoroughly saturate the biaxial cloth applied to cover and reinforce the hull seam on the stem.


When properly saturated with resin, the glass cloth becomes translucent. Tom Zydler


Here’s one of the worst spots we discovered, where the putty totally crumbled out of the seam near the propeller aperture. Tom Zydler


And here’s the repair: The seam in the hull along the propeller aperture has been glassed over and faired with epoxy thickened with colloidal silica. Tom Zydler


The seam in the stern has also been glassed over and faired with epoxy thickened with colloidal silica. Tom Zydler


Nancy is wet sanding to prepare the hull for the final application of the epoxy barrier coats. At this point all the glassing of the hull seam has been faired and the rest of the hull has also been faired and roughly sanded. The rudder is still protected from the drips of epoxy applied to the hull above it. Tom Zydler


The hull itself as well as the glass over the hull seam have been faired and sanded. Next come the epoxy barrier coats. Tom Zydler


Nancy rolls on the antifouling after the rudder has been barrier coated. The first coat of green will make it easy to see when the red antifouling on top of it wears out in future—a warning that another haul-out should be scheduled. Tom Zydler
What to do about boat blisters


Waiting for the poppets to be moved and the boat to be re-blocked in order to attend to these few remaining spots. Tom Zydler


I’m out of my “comfort zone” while filling gouges in the underside of the keel. Tom Zydler


On the way to the sea at last! Tom Zydler