Blister repair

We know that every other year our annual haulout will involve a little more than just sanding and painting the bottom of Alianna, our 1983 Corbin 39. While some might call our problem osmosis, we like to say that we just have a few blisters. Simeon and I knew there might be blister problems when we bought the boat five years ago, but we didn’t have time to wait for the hull to dry out so we could be sure. We wanted to go sailing. So far we’ve been able to manage the problem ourselves, and we plan to keep doing so until we have the time and money to do a more thorough job. Our solution isn’t a long-term fix, but it has kept the blisters under control.

What’s a blister?

“Osmosis” was the term commonly used around 30 years ago, though osmosis is not exactly what takes place when a blister forms. Simply put, water is slowly absorbed through an exterior gelcoat surface and diffuses into the fiberglass laminate, which may have small voids and/or tiny pinholes. When the water reacts with impurities in the resin in the fiberglass, it builds up a residue. The pressure created by the accumulating residue in these voids is what forms the blister you see on older fiberglass hulls. Seeing just a few blisters on a hull isn’t reason to panic, unless you’re trying to sell your boat. Otherwise, you can carefully monitor the situation, make repairs when necessary, and keep on sailing.

Identifying blisters

We always use boatyards that let us do our own work, so this is how we proceed. Ideally, we would remove all the bottom paint in order to see every blister on the bottom. But that’s time consuming, so we don’t take it quite that far. After we have hauled the boat and scraped the bottom, we put a stream of fresh water on the hull; the water that reflects off the hull surface helps to highlight the big blisters.

When we see one of them, we circle it with a colored pencil. Then we run our hands along the entire hull surface and usually find a few smaller bumps; these also get circled. Sometimes we see small amounts of moisture seeping out from the hull’s surface, and circle those too; they are definitely blisters. The liquid coming out of a blister usually smells like vinegar. We continue checking the hull’s surface until we locate all blisters that can be seen or felt.

Opening a blister

Blisters must be opened up so the water can drain out of them. To do this we use a grinder with a 60-grit flexible grinding disk and gently tease each blister. Then we widen each one out until there is no longer any visible dampness. The ground surface must extend down to the solid laminate.

The next step is to sand the hull to expose any blisters that may be hiding under the antifouling paint. When you’re sanding the surface, it’s important not to put too much pressure on one spot. When you do uncover a bump in the gelcoat, look closely at it because it might also be a blister. Ring those spots with the colored pencil and then take the grinder to them.

Once we are satisfied that we have exposed and ground out all the blisters, our next step is to flush all of them with fresh water to remove as much of the salty acid residue as possible. Spraying with a high-pressure hose for about 30 seconds will help to flush out a blister. We repeat the process every day for about a week, using a pressure washer if one is available.

Dry time

There’s no question that the longer the hull is allowed to dry, the better the repairs will be. This year we kept our boat out of the water for about 60 days. If the ambient air is hot but not humid, it’s easier for the hull to dry out. But no matter how long you leave the boat ashore, you should always leave an extra two weeks prior to your relaunch date to finish work on the bottom and allow for delays.

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