Fiberglass Blisters

Explanation, Diagnosis and Repair

The following is an attempt to demystify and clarify the fiberglass blister problem.

Past survey observations by Hartoft Marine Survey, Ltd. indicate that approximately 70% of the fiberglass vessels in the Chesapeake Bay area when inspected have fiberglass blisters to varying degrees of severity. This figure has recently been decreasing probably due to repairs having already been made. It is believed that approximately 95% or more of all fiberglass vessels in this area have, had, or will have, fiberglass blisters. Further observation shows that bottom laminate constructed with the commonly used orthophalic resins has a life expectancy of around 30 to 35 years before deterioration due to blistering and resin damage has structurally weakened the bottom laminate. This deterioration can often be observed as flexing of the bottom laminate when hand pressure is applied even on boats in the 40' to 50' range.

Some manufacturers have recently changed the resin types used in construction to higher quality resins (to isophalic or even better vinyl ester resins) in all or some of the outer layers of their vessels. Depending on resin type and application some of the comments above and below may not apply. Please consult with the builder for the exact specifications.

What is commonly referred to as a "Fiberglass Boat" is more correctly a "Fiberglass Reinforced Polyester Resin Boat". The hull skin is normally built in a female mold. First gelcoat is sprayed in the mold and then successive layers of different types of fiberglass (cloth, matte, roving, etc.) is layed into the mold one at a time and wetted out with polyester resin forming a hull consisting of many layers (or laminates) of fiberglass bound together with the resin.

Fiberglass blistering is caused by one or more factors such as resin type, contamination of materials, trapped gases, built-in voids, poor wetting out of laminate, incorrect humidity or temperature and dry layup. Osmotic fiberglass blistering is a process which depends on the temperature of, and exposure time to, the water. Given the above mentioned factors, it is not surprising that fiberglass blisters appear on a large number of vessels which are kept afloat for long periods of time in relatively warm water.

Fiberglass blisters form only when water penetrates to the laminate. This water not only damages the laminate by forming blisters causing localized delamination but also combines with uncured water soluble and hydroscopic components in the resin forming an acid solution which is highly corrosive to even the well cured polymers in the resin. As more water reaches the laminate, more corrosive solution is formed and more resin broken down. The effect is that of flushing the resin out from between the fiberglass strands. A laminate so affected is often referred to as having been hydrolyzed.

When a laminate gets hydrolyzed, a loss of strength takes place mostly in the form of increased flexibility. The hydrolysis is very layer specific and does not normally extend to the same degree through all layers. Consequently, the loss of strength can be quite high in the outer layer but the overall loss of strength in the hull itself minimal at least early on in the process.

Fiberglass blistering is a fairly rapid destruction of the outer layer of laminate (occasionally two or more layers) in the form of delamination due to the blister action. Typical blistering starts when the vessel is 5-10 years old (applying a barrier coating may delay the formation of blisters). All blisters will eventually break (unless the entire affected layer delaminates, a very rare occurrence) given the initial impression of a blister free bottom. Occasionally, blisters may then again form, this time further into the bottom laminate creating new and deeper blisters.

Hydrolysis (resin depletion) is a rather slow deterioration of the laminate (30-35 years) usually starting in the outer laminates and given enough time eventually affecting all laminates in the layup. Broken fiberglass blisters allow the water to reach the deeper layers of laminate more quickly, but hydrolysis often prevents blisters from forming in deeper layers due to the porosity caused by the resin depletion (in order for blisters to form, a membrane capable of holding pressure must exist).

Hydrolysis can be recognized by resin "wash out" between the fiberglass strands leaving a laminate that appears to be "dry", lacking resin. The resin remaining will be soft giving low barcol readings (below 40-45) and if the condition is severe, the bottom laminate may be easily deflected by hand pressure and will sound dull when percussion tested.

From the above, it follows that the most effective way to avoid blisters and laminate damage is to not expose the laminate to water. This can be accomplished by keeping the boat out of the water or if you want to use your boat, by applying barriers to slow down water absorption. If the boat is new and has never been in the water, a barrier coating can easily be applied. If the boat is used and maybe even has blisters, things become more complicated. Please note that hauling a vessel for winter storage to "dry out" will not significantly, if at all, lower the trapped moisture in the laminate unless all gelcoat has been removed.

The objective of the fiberglass blister repair is to preserve or improve the current structural integrity and to prevent any future significant structural weakening of vessel's bottom laminate due to the fiberglass blistering. The objective must also be to balance the life expectancy of the repair and the boat against the cost. It is believed that with the materials and techniques available today, a good barrier coating, under normal circumstances, should last four to eight years and a proper performed relamination may last for the life of the boat.

Cost is a serious consideration for most boat owners and in most cases spending money below the waterline instead of above the waterline on cosmetics and equipment is not a very attractive proposition except possibly for a "racing bottom". Barrier coating is a way of preserving "status quo" and preventing additional damage for a limited period of time, a "temporary repair". Relamination is a repair of existing damage and a way of restoring or maybe even improving original structural integrity. With the right materials used, this is considered a "permanent repair". Relamination costs approximately 1.5 to 2 times as much as barrier coating and is becoming as a very attractive alternative to the old way of repairing with a barrier coating. This is particularly true when considering that relamination also addresses hydrolysis and that the repair often can be delayed for the normal life span of 1 to 2 barrier coat repairs without significantly affecting the vessels structural integrity.

Blisters below the waterline can be divided into three groups: "paint blisters", blisters in earlier applied protective coating or antifouling paint; "gelcoat blisters", blistering of gelcoat with no involvement of laminate; and "fiberglass blisters", blisters involving one or more layers of laminate.

Paint blisters can be considered purely cosmetic and should be treated as such. Blistering of earlier applied protective coating would indicate that the coating is compromised and would need to be renewed for best possible protection of the laminate keeping the objective of the fiberglass blister repair in mind. Coating blisters are 1/16 inch to 1 inch in diameter, most common are 1/16 inch to 1/8 inch in diameter. Blisters can be punctured by light finger pressure and contain a sticky acid fluid (always wear eye protection when opening any kind of blister).

Gelcoat blisters with no involvement of laminate is rarely seen but when seen is most often found to be one layer of gelcoat blistering off another underlying layer of gelcoat. This type of blistering is mostly cosmetic, but can possibly increase the likelihood of fiberglass blisters developing. Gelcoat blisters are 1/16 inch to 1 inch in diameter, most common are blisters 1/8 inch to 1/4 inch. Blisters can be easily punctured with a knife tip. Blisters are "crunchy" and contain a sticky acid fluid.

Fiberglass blisters involving gelcoat and part of first layer of laminate is by far the most commonly observed type of blistering. This type of blistering causes relatively little structural damage initially and repairs can often safely be postponed for several years. However, a wet laminate is substantially less strong than a dry laminate and moisture penetration into the laminate can remove resin between fiberglass strands, accelerating the weakening of the structure over time. Blisters are 1/8 inch to 1 1/2 inch in diameter, most common are blisters 1/8 inch to 1/4 inch in diameter. Some pressure is needed with a knife tip to puncture the blister. Blisters are "crunchy" and contain a sticky acid fluid.

Fiberglass blisters involving one or more layers of laminate leads to rather rapid loss of the structural strength of a vessel's laminate depending on number of blisters and layers of laminate involved. This type of blistering normally calls for corrective action within a relative short time span (1 to 3 years). Blisters are 1/2 inch to 3 inches in diameter, most common are blisters 1/2 to 2 inches in diameter. Blisters can be cut using a knife applying some force. Blisters contain a sticky acid fluid.

Before a repair is undertaken, the laminate should be inspected to determine to what degree the laminate is damaged by blistering and hydrolysis. A "window" should be created in the bottom laminate. This is done by grinding into the laminate layer by layer until good laminate is found. Each layer is measured for moisture, hardness (barcol) acidity and thickness. Layers are also visually inspected for resin and blister damage. Thickness of damaged layers can be expressed as a percentage of overall skin thickness. When the information has been analyzed, a decision can be made as to whether any repairs are needed at the present time. If a repair is deemed necessary, a decision has to be made as to barrier coating or relamination taking into consideration the use of the vessel and expected term of ownership verses cost.

A fiberglass blister repair commonly consists of an applied barrier coating to prevent the water from entering the vessel's bottom laminate. None of the presently used barrier coats are 100% water tight and water can, molecule by molecule, penetrate through the created membrane into the vessel's laminate. Microscopic cracking of the barrier coat may also occur due to lack of reinforcement from fiberglass strands again causing water to reach the laminate. Consequently a fiberglass blister repair using barrier coating is not a permanent repair but needs to be renewed after a period of time.

Based on experience and observations, the following basic steps may be taken to assure a satisfactory fiberglass blister repair with maximum life expectancy. The below outlined procedure is for illustration purposes only and may vary between repair facilities.

1. All gelcoat should be removed from below the waterline. This prevents blistering from reoccurring in the interface between the gelcoat and the first layer of laminate, by far the most common area where blisters are seen. The removal of gelcoat also promotes drying of the laminate.

2. The vessel's bottom laminate should then be dried to an even moisture content, comparable to that of the vessel's topsides. On a Sovereign Moisture Master Meter, moisture content should not be more than 5% on the A scale (this is not the actual moisture percentage in the laminate, readings are for comparative purposes only). The reason for drying is two fold. The less moisture that is trapped in the laminate, the more water molecules are needed to pass through the barrier coating before enough pressure builds up to form new blisters. If the laminate moisture content is too high, many of the barrier coatings used will not cure or adhere properly. This often results in blistering of the barrier coating itself or total failure of the bonding of the barrier coating.

3. Wash the laminate after drying to remove any solids and contaminates that might have reached the surface of the laminate after the drying process. This washing can be plain water or an alkaline solution. If an alkaline solution is used, Ph testing should be performed after washing to make sure the alkaline solution has been completely removed. Laminate should be dry before proceeding.

4. The entire below waterline surface should be sealed with one to two coats of thin resin. This resin should penetrate between any loose fibers and into any pores left in the surface of the laminate from removal of the surface, helping to fill and seal as many voids as possible and to create a good bond for the products subsequently applied. The bottom should then be faired with a compound containing non-water absorbent filler.

5. When fairing is completed, barrier coating should be applied to the bottom following the product manufacturer's instructions; however, no less than three coats (not counting coats applied before fairing) should be applied whether the coating is brushed, sprayed, or rolled. Anything less than three coatings will not guarantee complete mechanical coverage. To insure sufficient coat thickness throughout the vessel's bottom, it is suggested that not less than five coats be applied with more being better. If sanding is used or called for between coats, the number of coats may have to be increased.

A more radical and in general much longer lasting approach to repairing fiberglass blisters is to remove all damaged bottom laminate, then laying up new layers of laminate using epoxy or vinylester resin (laminate made with both of these resins have been shown to be much less likely to blister than polyester resin). This repair method has now been proven for a sufficiently long time to almost be labeled as a permanent repair. A theoretical possibility exist of blisters eventually forming in the interface between the different resin types or in the old laminate. Relamination of vessel's bottom is in general only suggested by Hartoft Marine Survey, Ltd. when fiberglass blistering is seen involving more than one layer of laminate or when severe hydrolysis has taken place.

The procedure for relamination follows that of barrier coating closely except for laying up of laminates as needed between step 4 and 5, however then the fairing step is often postponed until after the lamination is completed.

When a decision has been made to repair a vessel's bottom a number of additional considerations are needed. The approach to the repair as suggested by the chosen repair facility should be checked against the above outlined procedure and a resin/coating system should be selected. It is strongly recommended that the resin/coating system to be used is one with which the repair facility is familiar and has a positive experience in applying. A large number of barrier coating systems from different manufactures are available today; however, reliable records of effectiveness of the repairs made in the field with any of the systems are poor, making specific suggestions difficult.

A number of the resin systems are well documented and supported by their manufactures. Not all of the available products are compatible. It is strongly suggested that products from one system not be mixed with products from another system unless thorough testing is first performed. Of all the systems available, the epoxy resin systems appears in theory to be the most desirable. It should be pointed out that epoxy systems require very controlled environments and rather precise techniques during application, which might turn out to be a very limiting factor when used in the normal boat yard environment. Vinylester resins have been gaining popularity as a barrier coating and as a resin for use in relaminating, due to their ease of working and their much reduced sensitivity to environmental factors, they appear to work very well in the boat yard environment but under laboratory conditions they do not show quite the outstanding characteristics of the epoxy resins. The failure rate of vinylester resins is at present by far the smallest of the commonly used materials making their use very attractive.

Regardless of the system used it is suggested that to achieve the best possible curing and to lessen the chance of contamination, a controlled environment should be created around the vessel when coatings are applied. This can be in the form of a plastic tent with temperature and humidity control or the vessel can be placed inside a controlled building. Again, it should be pointed out that a repair facility should be chosen which is familiar and experienced with the desired coating product and procedure.

There appears to be no advantage to treating fiberglass blisters on an individual bases except possible pressure relief by puncturing blisters. Any other approach can at best be considered cosmetic only. When fiberglass blisters are observed on a vessel's bottom, a determination should be made as to the severity of the blistering. If only a few scattered blisters are seen in the bottom involving gelcoat and portions of first layer of laminate it is suggested that no immediate corrective action be taken but that the blister condition be monitored at subsequent haul outs by the "window" process.

If fiberglass blisters are observed involving two or more layers of laminate or severe hydrolysis has taken place, it is suggested that the overall structural condition of the vessel be evaluated and balanced against the use (i.e. "inshore" or "offshore") and corrective measures be taken within a reasonable time period as outlined above.



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