Fiberglass Boat Blisters and Osmosis: What a Marine Survey Shows
Introduction
Blisters are a common observation during haul-out inspections of fibreglass boats. Although they are not generally the most serious structural problem on a boat—faulty construction, de-bonding, or flaws in the materials could be more important—they do have an effect on the boat’s value, on buyers’ confidence and on the cost of repairs. The marine surveyor’s responsibility is to identify blisters, describe them objectively and report them clearly so that the client knows what steps to take next.
The chapter deals with the causes, the way osmotic blistering develops, how it should be inspected and recorded, examples of condition statements, and the treatment alternatives that owners usually consider. It is intended for use in Condition & Value (pre-purchase and insurance) surveys of recreational fibreglass vessels.
How Osmotic Blisters Form
A typical fiberglass hull is coated with gelcoat which is applied over layers of glass reinforcement and resin. Although gelcoat resists water it does not form a perfect waterproof barrier and as a result moisture can therefore seep into the gelcoat. When this moisture comes into contact with water-soluble compounds in the laminate or at the interface between the gelcoat and the laminate, a concentrated solution is formed. Osmotic pressure then causes more water to be drawn in and results in the formation of blisters that are filled with fluid and which appear either between or within the layers.
Useful terms:
Permeable / permeate – the capacity of a fluid to go through the pores or spaces of a solid.
A semipermeable barrier is one which permits certain molecules (typically water) to pass through while preventing others from doing so.
Osmosis is the process whereby a fluid diffuses through a semipermeable membrane due to differing concentrations on either side; for osmosis to occur, there must be permeability and solutions on both sides of the barrier in the ordinary sense; in the case of hulls, the actual result is the formation of fluid-filled blisters either under the gelcoat or within the laminate.
Hydrolysis — a chemical reaction in which water breaks down a substance into new compounds; this process is relevant to the degradation of resin over time.
Capillary effect — Tendency of fluid to move through narrow passages; related to absorption, diffusion, and permeability.
Blisters occur most frequently below the waterline since it is there that the duration of immersion and the pressure are greatest.
Common Causes
Many blister problems start in construction or materials, then worsen with immersion:
Poor workmanship or inconsistent lamination
Inadequate wetting of glass with resin (voids and dry spots)
Over-catalyzed resin or poorly mixed catalyst (uneven or brittle cure)
Excessive time between gelcoat and laminate application (weak bond)
Unstable additives, fillers, or pigments
Pinholes or defects in gelcoat that admit water
Resin chemistry that is more susceptible to hydrolysis / osmotic attack
Surveyors should treat blisters as a finding and as a possible clue to broader laminate or construction quality—without claiming a cause that a visual survey alone cannot prove.
How We Inspect
Blister inspection is an out-of-water task. During haul-out:
Carry out a visual inspection of the entire area below the waterline (and make a note of any blistering above the waterline, since this is less common and might have different causes).
2. To check the hull bottom and any areas that are suspected, percussion should be carried out and listen for returns that are either dull or hollow, as these could be a sign of de-bonding or laminate problems related to blister fields.
When using a moisture meter, take readings consistently on a regular grid after having noted how long the vessel has been hauled; recently hauled hulls tend to show higher moisture levels, so the readings should be interpreted in context and a single high reading must never be taken as evidence of structural failure.
4. Record the range of sizes, the distribution (whether it is scattered or concentrated), the estimated percentage of the area under the water that is affected, and the approximate depth when this can be judged (in the case of superficial gelcoat involvement versus deeper laminate involvement).
If the yard opens a blister in order to carry out an assessment, it should be noted whether the contents are wet or dry.
Unless the client has asked for the blisters to be inspected in a repair yard, do not bother grinding, cutting or opening them and let the surveyors report what they see and carry out the tests.
Things to Look For
Pattern: blisters that are isolated as opposed to those that are widespread, known as ‘boat pox’
Size: dime-sized up to several inches
Depth/hardness: soft, fluid-filled vs. hard / previously repaired
Prior repairs: fairing, epoxy patches, barrier coat history
Soft laminate, voids, or delamination near blister fields
Correlation with high moisture readings after adequate dry time
Bottom paint condition and whether blistering is under antifouling only or in laminate
What to Report
Give concrete examples. Do not use general expressions such as “some blistering” or refer to an undefined “cosmetic condition.”
Include:
Location (e.g., port amidships bottom, full bottom below waterline)
The approximate percentage of the area submerged below the waterline.
Size range
Where possible, classify by depth or severity (for example, superficial or deep / laminate)
Moisture observations if taken
Photos keyed to findings.
The standard approach to preparing Condition & Value reports focuses on presenting clear findings that are centered on hazards and includes references to voluntary standards such as ABYC and NFPA 302 where these apply to various systems; in the case of hull blister reporting, the assessment is mainly based on professional judgment and supports this with visual, sonic, and moisture testing methods. When assessing the severity of a condition and making recommendations, surveyors should apply their own judgment taking into account the level of hazard and the client’s purpose (whether the aim is to purchase the vessel or to obtain insurance).
Sample Condition Statements
Isolated/mild (observation only):
Blisters in the gelcoat were seen on the underside of the hull, below the waterline, mainly in the middle section. The size of these blisters varied from about the size of a dime up to ¾ inch. The estimated area affected was under 5% of the hull surface below the waterline. The percussion tests carried out on the surrounding laminate were in agreement with those of the adjacent regions. Moisture meter readings were taken following haul-out and are listed in the findings. At the time of inspection, no soft laminate was found in the blister area by means of sounding. RECOMMENDATION: It is advised that a qualified marine fibreglass technician inspect the blistered areas (including carrying out moisture mapping after sufficient dry time) and then recommend the appropriate repairs before making the final decision on purchase or insurance.
Moderate / recommend further evaluation:
A large number of blisters were seen on the lower part of the hull underneath the waterline, the size of the blisters varying from about dime-sized up to around 3 inches. About one quarter of the area below the waterline was affected. Some of the blisters gave a soft sensation when lightly pressed upon. The moisture meter showed high readings in a number of places. The percussion tests produced dull sounds in certain areas near the groups of blisters. Recommendation: It is advised that a qualified marine fiberglass technician examine the areas affected by blisters (including carrying out moisture mapping after sufficient drying time) and then recommend the necessary repairs before making the final decision on purchase or insurance.
Widespread/significant:
Blistering was seen in extensive areas of the hull beneath the waterline, the blisters varying in size from about half an inch up to more than three inches. The area affected was estimated to be more than 50% of the section of the hull below the waterline. In several places, soft blisters containing fluid and dull sounds when tapped were observed. Recommendation: It is advised that a qualified marine fiberglass repair workshop assess the hull for osmotic blistering and laminate condition and should issue a written repair plan together with an estimate. Final purchase decisions should be postponed until this assessment has been carried out.
When carrying out a survey, state what was observed or tested, and then provide a clear recommendation if it is appropriate—to do so without suggesting a particular way of carrying out the repair or a specific contractor.
Owner Options (Report Facts, Don’t Sell a Repair)
Owners typically choose between:
1. For localized or spot repairs — grind the individual blisters, dry them, fill them (typically with epoxy), smooth the surface, and then refinish the area. This method has a lower near-term cost; it may be appropriate for cases involving only isolated blistering; however, there is a risk of recurrence if the underlying problems still exist.
2. Full bottom/peel and barrier system involves removing the gelcoat and the outer layers as required, drying the laminate, reassembling it, and then applying a suitable barrier system. Although this option is more expensive, it is generally chosen when blistering is extensive and long-term ownership is intended.
Surveyors present the condition and implications; the client and the repair yard to decide on the method. The costs differ greatly depending on the size of the vessel, the access available, and the rates charged by the yard.
Summary
Osmotic blistering is the formation of blisters in fiberglass hulls as a result of moisture, and this usually occurs below the waterline; the causes are generally connected with the quality of construction and the materials used. When inspecting, do so when the vessel is out of the water using a visual survey, sounding, and a careful analysis of the moisture condition. You should give details about the size, position, degree of spread, and how severe the blisters are in measurable terms, include photographs to support your report, and suggest a further technical evaluation if the findings go beyond being merely isolated and superficial.
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