Patch Test: Blisters Need Not Be A Deal-Killer With Proper Evaluation - PassageMaker

Patch Test: Blisters Need Not Be A Deal-Killer With Proper Evaluation

Let’s imagine that the boat has been hauled, and a dozen or so blisters can be observed. How do you determine the severity of the problem? Should you walk away, or perhaps renegotiate in hopes of lowering the price?
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Every pre-purchase survey includes inspection of the vessel’s bottom for a variety of problems, including blisters. Let’s imagine that the boat has been hauled, and a dozen or so blisters can be observed. How do you determine the severity of the problem? Should you walk away, or perhaps renegotiate in hopes of lowering the price?

Much has already been written about the causes and cures for osmotic blisters and need not be covered again here. But this much should be stated: Blisters are not the problem—they are symptoms of the problem. They are the cough, but we need to know if the boat has a mild cold or pneumonia. Opening the blisters and filling them accomplishes the equivalent of a cough suppressant—it makes you feel better but doesn’t cure the illness.

Too often, the appearance of blisters is dismissed with statements such as “It’s a boat, they all get blisters” and “We can open them up and fill them in.” Such statements reflect a lack of understanding about the true issue at hand. In order for blisters to form, moisture must penetrate the underlying laminate. When this happens, a critical question must be answered: How deep into the laminate has the moisture migrated?

In the mildest cases, the moisture might be just beneath the gelcoat at the first layer of fiberglass material.

The milky appearance of this laminate indicates that moisture has done some damage.

The milky appearance of this laminate indicates that moisture has done some damage.

In a moderate case, the moisture might be between the first and second, or second and third layers of fiberglass.

In the more severe cases, the moisture might have worked its way all the way into the structural laminates, perhaps 1/8 inch deep or more. In order to make an informed buying decision, you must know the answer to this question.

MOISTURE METER?

You might be thinking that a moisture meter can answer this question, and you would be partially correct. Although a moisture meter should be considered an essential tool for every survey, it has its limits. In a pre-purchase situation, the first limit is time.

After removing layers of outer-hull material, a moisture meter will indicate how deep moisture has penetrated.

After removing layers of outer-hull material, a moisture meter will indicate how deep moisture has penetrated.

Most often the boat will be hauled the day of the survey. The bottom paint will retain enough moisture to render the meter useless below the waterline. If the boat has been out of the water for a week or more, the meter will tell us if moisture has soaked into the laminate, but will not answer the critical question of depth.

To find the answer, a patch test must be performed. A patch test begins with removal of the bottom paint in a small area no larger than the footprint of the moisture meter.

On a 40-foot boat, I would recommend testing six areas: forward, amidships and aft, a foot below the waterline, halfway down, and just above the keel. Any areas with blisters should be included

The patch test proceeds by progressively removing one layer of material at a time and taking moisture readings at each step. This process continues until the readings are acceptable (“acceptable” depends on the type of meter and other factors—a good surveyor or fiberglass technician will know the right thresholds).

In addition to the moisture readings, look for another critical piece of information—the visible condition of the laminate. Wiping the bare laminate with alcohol will enable you to see “into” it. A healthy laminate will be dark and somewhat clear. If the laminate is milky with yellowing and/or whiteness, moisture has found its way in. Invariably a milky laminate will also ping the moisture meter. This cloudy laminate, referred to as “hydrolyzed,” speaks to the crux of the blister issue.

The fiberglass that gives your boat its strength and shape consists of a matrix of resin and fabric. The fabric looks white when it is dry, and turns clear when the resin is applied. When moisture finds its way into the laminate, it reacts with residual chemicals and becomes acidic. This acidity dissolves away the resin, returning the fabric closer to its original white appearance. The lighter color tells us that resin is gradually leaving the laminate.

Although the hydrolyzed laminate is weakened, structural concerns are rare. Left untreated, however, the moisture will continue more deeply into the laminate, increasing the repair cost and devaluing the boat. While wet laminate is rarely a structural concern, it is always a resale/value concern.

PRICE IT

The patch tests will answer our question about the extent of the moisture penetration by clearly identifying the condition of each layer of affected laminate. Once this information has been gathered, a detailed cost estimate to repair the problem can be prepared.

In a pre-purchase situation it might be possible to adjust the purchase price by some factor of this cost. A boat with a properly executed blister repair job has additional value and years of protection against a recurrence, and for those reasons some sharing of the cost between buyer and seller would be appropriate.

Some buyers walk away at the sight of blisters on the bottom. While that is certainly a viable choice, if the price can be adjusted appropriately, in some ways I would prefer a boat with a proper blister repair and barrier coat over a boat that has no blisters. With a proper repair job, you will have years of known protection, which translates to added value if you sell the boat. 

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