Hull blisters on a Mobile Bay boat: what they mean and when to fix them

Most of the boats we see with blisters have spent years in the water on the western shore, in the Fowl River slips, or behind a house on the Eastern Shore. Warm brackish water twelve months a year is the setup for it. The good news is that blisters are slow, they are visible, and they almost never surprise you the way a wet transom does.

What a blister actually is

Gelcoat is not fully waterproof. Over years afloat, small amounts of water pass through it and reach the laminate underneath. Inside the laminate there are water soluble leftovers from the original layup, and when water finds them it forms a fluid that pulls in more water. Pressure builds under the gelcoat and lifts a dome. That is the blister. Pop one with a screwdriver and you usually get a spurt of fluid with a sour smell, which is the giveaway that it is osmotic and not a paint problem.

Bottom paint blisters are the thing people confuse them with. Those sit in the paint layers, are dry when opened, and peel off with a scraper. They matter for the paint job, not for the hull. The first thing we do at the boat is open one bump and see which kind you have.

A few blisters versus a bottom problem

Count is the obvious measure but it is not the important one. What we care about is depth and moisture. A handful of shallow blisters that stop at the first layer of mat are a local repair, and we treat them the same way we treat a small hull section repair: open, dry, grind, fill with the right resin, fair, and refinish. That is a job you can schedule around your season.

A bottom that is peppered from the waterline down, or blisters that run deep into the laminate, or a moisture meter that reads high across whole panels, means the water is no longer in a few spots. It is in the hull. At that point patching individual blisters is a waste of your money, because new ones will keep coming up around the repairs. The right job is opening the bottom up, drying it, and putting a barrier coat over everything.

Why Mobile Bay boats get them

Warm water moves through gelcoat faster than cold water, and our water is warm most of the year. Boats that never come out, which describes a lot of the Dog River and Fowl River fleet, sit in that condition without a break. Brackish water is a little gentler than open Gulf salt, but a sailboat or cruiser that has lived in the river for a decade or more has had plenty of time regardless. Older hulls also tend to have been built with resins and gelcoats that resist water less well than what goes into a new boat, so age counts twice.

Trailered boats and boats on lifts rarely blister. If you keep your bay boat dry between trips, this is probably not your problem, and a bump you found is more likely a paint issue or an old impact.

What the repair involves

For a small number of blisters we open each one with a grinder, wide enough to get past the wet edge, and let it dry. Then we fill with an epoxy or vinyl ester fair compound, sand it flush, and put the bottom paint back. The step most people skip is the drying. Filling a blister that is still wet traps the problem under new material and it comes back.

For a widespread case the sequence is bigger. The bottom paint and gelcoat come off the affected area, the hull is left open to dry, and we check moisture readings over time until they come down and stay down. In our climate that can take weeks, and longer on a hull that has been in the water for many years. Only then does the laminate get refaired and sealed under an epoxy barrier coat, followed by fresh bottom paint. The barrier coat is the part that slows water getting back in, and it is the reason a full job costs what it does.

We do not publish a number for blister work because hull size and how wet it is decide almost everything. Our cost guide lists it as quoted after inspection for that reason. What we can tell you at the boat, after opening a couple and metering the bottom, is which of the two jobs you have and a written range for it.

When to leave them alone

We tell people to leave blisters alone more often than you might expect. A few small ones on a boat you are keeping for years are not urgent. They do not sink boats, and they grow slowly. If the boat is coming out for bottom paint anyway, fix them then. If it is not, they can wait for the next haul-out.

The exception is a boat you are selling. Buyers and surveyors look at the bottom first, and a row of blisters turns into a negotiation. Whether that repair pays for itself depends on the boat, and we will say so if it does not.

Insurance and blisters

Blisters are treated as wear, not damage, and policies we have seen exclude them. Impact and storm damage are a different matter, covered in our guide to checking a fiberglass boat after a storm. If you are not sure which one you are looking at, a photo usually settles it.

What to send us

Take a photo of the worst area with something for scale, a wider shot of the whole bottom if the boat is out, and tell us how many years it has been in the water and where. From that we can usually say whether it sounds like a local repair or a bottom job, and whether it is worth doing before the boat goes back in. Reach us at (251) 220-4157.