When 100% Moisture Isn't Moisture: How Antifouling Can Mislead a GRP Hull Survey
A high moisture reading does not automatically mean osmosis
One of the most important lessons in GRP yacht surveying is that a moisture meter is a diagnostic tool—not a crystal ball.
Recently, I was asked to carry out an underwater hull survey on a GRP vessel after a previous survey had reported extremely high moisture readings throughout significant areas of the laminate.
The readings were reportedly between 80 and 100 on the GRP scale.
Those numbers naturally raised concerns about osmosis and the possibility of an expensive osmosis-treatment programme.
Before committing to such treatment, however, I was asked to carry out an independent inspection and verify the findings.
What I found was a good reminder of why moisture-meter readings must always be interpreted in context.
The initial readings
The first survey had identified very high readings on the underwater part of the hull.
On a moisture meter such as the Tramex Skipper 5, readings on the GRP scale can range from 0 to 100. A high reading can be an indication of elevated moisture, and potentially of conditions associated with osmosis.
But there is an important distinction:
A high meter reading is an indication that requires investigation. It is not, by itself, a diagnosis of osmosis.
Tramex itself advises that when a Skipper 5 produces high readings, the surrounding area should be examined for blistering or other evidence of osmosis.
That distinction became particularly important during this inspection.
Looking beyond the number
I started with a physical examination of the underwater hull.
The hull was systematically tapped using a phenolic hammer to look for changes in sound that might indicate problems such as delamination or areas requiring further investigation.
At the same time, I looked for the physical signs that would support an osmosis diagnosis.
There were no obvious widespread osmotic blisters.
There was no convincing pattern of laminate deterioration corresponding to the extremely high moisture readings.
And then came the most interesting part of the investigation.
Scraping away the antifouling
Rather than simply accepting the meter readings, I selected a number of random locations and carefully scraped away the antifouling coating.
I then repeated the moisture measurements directly on the exposed surface.
The result was striking.
The readings dropped substantially.
In locations where the antifouling remained intact, the Skipper 5 could show extremely high readings.
After removing the coating, the same general areas could produce dramatically lower readings.
That raised an obvious question:
Was the meter actually detecting moisture in the laminate—or was it responding to the properties of the antifouling coating?
Why can antifouling produce high readings?
The answer lies in how a non-invasive moisture meter works.
The Skipper 5 does not simply reach into the laminate and measure the amount of water present.
According to Tramex, the instrument applies a high-frequency electrical signal between electrodes on the underside of the meter. The instrument measures the electrical impedance of the material and interprets this response as a moisture value.
GRP itself is normally non-conductive, but its electrical properties change as moisture content changes.
The complication is that other materials can also influence the electrical response.
This is particularly relevant to antifouling coatings.
Many marine antifouling paints contain conductive pigments or other conductive components. Copper-based antifouling is an obvious example because copper compounds are deliberately incorporated into many antifouling formulations.
A conductive coating can therefore interact with the electrical field used by a capacitance/impedance-type moisture meter.
The instrument does not necessarily know whether the change in electrical response is caused by water in the laminate or by a conductive coating sitting on top of it.
And this is not merely a theoretical possibility.
Tramex explicitly warns about antifouling
The Skipper 5 user guide contains a very important warning:
“Some anti-foulings may give elevated readings due to the presence of conductive material in its composition.”
Tramex recommends removing the antifouling and carrying out further testing when this occurs.
This is exactly why the practical test of removing the coating was so important in this case.
Copper-based antifouling: the likely culprit
It would be tempting to say that the copper in the antifouling “tricks” the moisture meter.
That is broadly understandable, but technically it is better to be more precise.
The Skipper 5 is responding to the electrical characteristics of the system beneath the instrument, not independently identifying water molecules.
A coating containing conductive material can alter the electrical field and impedance seen by the meter.
As a result, the instrument can display a high comparative GRP reading even though the underlying laminate may not contain the amount of moisture that the number might initially suggest.
In other words:
The meter may be reporting a real electrical difference—but the interpretation of that difference as laminate moisture can be wrong if the coating is influencing the measurement.
This distinction is extremely important for surveyors.
Why the 80–100 reading was misleading
A reading of 80, 90 or 100 can look alarming.
It is very easy to see such a number and immediately think:
“The hull is saturated.”
But that conclusion skips several steps.
A proper interpretation should instead be:
“The instrument has detected an unusually high electrical response. Now I need to establish why.”
That means looking at the pattern of readings, the construction of the hull, the condition of the coatings, the location of the readings, and—most importantly—the physical evidence.
Tramex itself emphasizes that it is the overall trend that matters and notes that unusually high readings can have several causes, including structural elements and conductive materials.
The importance of comparative testing
One of the most useful techniques during a moisture survey is comparative testing.
Instead of asking only:
“What number does the meter show?”
ask:
“What happens to the number when the conditions change?”
In this case, the experiment was straightforward:
Measure the hull with the antifouling intact.
Identify areas producing unusually high readings.
Remove the antifouling from selected test locations.
Measure the exposed surface again.
Compare the results.
Correlate the readings with the physical condition of the laminate.
The dramatic change in readings after removing the antifouling provided important evidence that the coating was influencing the measurement.
This is one of the reasons why a moisture survey should be considered an investigative process, rather than simply a collection of numbers.
Moisture meters are incredibly useful—but they have limitations
The Tramex Skipper 5 is specifically designed to help surveyors identify excess moisture in GRP structures and can measure at different depths. The deep GRP mode can detect conditions deeper within the laminate, while the shallow mode provides a more localized measurement closer to the surface.
That makes the instrument extremely useful.
But no non-destructive moisture meter should be treated as an instrument that can diagnose osmosis by itself.
A high reading should trigger further investigation.
It should not automatically trigger an expensive treatment programme.
The meter should be one part of the evidence.
The rest comes from the hull's construction, history, physical condition, coating system, moisture distribution and other survey findings.
What happened in this case?
After examining the hull and carrying out comparative testing, I could not find physical evidence corresponding to the extremely high moisture readings that had initially suggested a serious osmosis problem.
The most interesting discovery was that areas covered by the antifouling produced very high readings, while scraping away the coating resulted in substantially lower readings.
That observation strongly suggested that the antifouling system was having a significant influence on the meter readings.
This does not mean that every high reading under antifouling is automatically false.
Nor does it mean that the hull should be declared dry simply because the reading falls after removing paint.
It means something more important:
The coating has to be taken into account before the readings can be interpreted as evidence of moisture in the laminate.
A useful lesson for surveyors and boat owners
This case demonstrates a principle that applies well beyond one particular boat:
Never diagnose osmosis from a moisture-meter number alone.
A moisture meter can tell you that something is electrically different.
Your job as a surveyor is to determine why.
If the meter shows 90–100 on an underwater GRP hull, don't immediately assume that the laminate is saturated.
Ask:
Is the antifouling influencing the measurement?
Is the coating conductive?
What happens when the coating is removed?
Are there osmotic blisters?
Is there evidence of laminate deterioration?
Is the pattern consistent across the hull?
Are there structural elements underneath the measurement area?
What do shallow and deep measurements show?
Are there comparable areas with different coating conditions?
Has the instrument been checked for calibration?
Tramex recommends checking the Skipper 5 calibration before commencing an inspection, using its calibration check box.
These questions are much more useful than simply recording the highest number on the dial.
Don't spend thousands based on a number
Osmosis treatment can involve substantial preparation, stripping, drying time, labour and recoating.
That is why the diagnosis needs to be right before treatment begins.
If a boat owner is told that their hull has 80–100% moisture and therefore requires an expensive osmosis treatment, it is reasonable to ask how that conclusion was reached.
Was the antifouling removed?
Was the hull physically inspected?
Were the readings compared in different areas?
Was there evidence of blistering or laminate degradation?
Was the meter properly calibrated?
Were the readings interpreted in the context of the particular coating system?
These questions can make the difference between identifying a genuine structural problem and treating a problem that exists primarily on the surveyor's moisture meter.
The bigger lesson
The most valuable tool in a marine survey isn't necessarily the most sophisticated instrument.
It is the ability to question what the instrument is telling you.
In this case, the moisture meter produced numbers that initially looked extremely concerning.
But once those numbers were challenged with physical inspection and controlled comparative testing, a very different picture emerged.
The antifouling itself appeared to have a major influence on the readings.
And importantly, this is consistent with the manufacturer's own warning that conductive antifouling can cause elevated readings and that removing the coating is an appropriate way to investigate the result.
So the next time you see a GRP hull producing spectacularly high moisture readings, remember:
A high number is a reason to investigate—not a reason to panic.
And sometimes the most important thing you can remove from a hull survey isn't moisture.
It's the paint.