fiberglass blisters and barrier coats · osmosis. unfortunately, this doesn’t help all the boats...

38 OCEAN NAVIGATOR No. 136 March/April 2004

Editor’s note: In the first of this two-part series on the sub-ject of fiberglass blisters, we’ll explore the basics of pro-duction boatbuilding, different types of resin, glass fab-rics, and boatbuilding techniques, and the role they playin the formation of fiberglass blisters.

In the 16 years that I’ve worked in and managed boat-yards, I’ve encountered few repair subjects that strikeas much fear into the hearts of boatowners as that of

hull blisters.To an extent, their fear is well placed; hull blis-ters, sometimes referred to as osmosis, are a serious prob-lem that may, under some circumstances, weaken a vessel’sfiberglass laminate. However, one thing is certain:A case ofhull blisters will compromise the marketability and value ofmost boats, just ask any broker.The validity of this devalua-tion is and will, no doubt, remain fertile ground for debate,primarily because experts continue to disagree about justhow much osmosis weakens a laminate.Additionally, in myexperience, the degree of compromise varies widely fromboat to boat.

Researcher bewareThe impetus for this article stemmed from a correspon-

dence piece published in these pages several months ago(Barrier-coat problems require extended fix Issue 132,Sept./Oct. 2003).A voyaging couple, owners of an afflicted

boat, lamented the dearth of information concerning thecauses of and solutions to osmotic blistering.Before delvinginto this subject, let the reader be warned. In reality, nosuch scarcity of information exists on the subject of osmo-sis causes and repairs, much of which can be found on theInternet. Sadly, a great deal of this information is flawed,inaccurate or purely anecdotal.

Remember, anyone can create a Web page or postauthoritative-sounding tomes on this and many othermarine subjects. Consider the source — books and articlespublished by experienced, respected experts are usuallyvetted by equally experienced editors, while most websitesare not.This is not to say that there isn’t a great deal of accu-rate information available on the subject of osmotic blis-ters, which may be found on the Web. In fact there is, andmuch of it originates from university researchers, chemicalengineers and manufacturers of fiberglass resins and com-posites.

Simply put, if you are faced with a case of fiberglass blis-ters, research carefully and resist the temptation to acceptsolutions solely on the basis of expense, rather than expert-ise and techniques with a proven record of success.

The fiberglass-reinforced polyester (FRP) constructionprocess uses several basic and a few complex components.Once again, in order to understand how fiberglass blisters

Coating Technology

Story and photos by Steve C. D’Antonio

Antifouling paints appliedwithin the “chemical

window” of certain epoxybarrier coats adhere

tenaciously to this surface,making for effective and

long-lasting coatings.

Fiberglass blisters and

barrier coats

Fiberglass blisters and

barrier coats


Coating Technology

form, it is necessary to understand these terms and materi-als.

The aforementioned gelcoat is simply pigmented resin.In the early days of FRP boatbuilding, many manufacturersthought that gelcoat would provide an impermeable barri-er to water (more on why this is important later), althoughfew had any idea of the importance of this feature at thetime. Additionally, it was also billed as being so hard andslick that barnacles would be unable to keep their grip,thereby eliminating the need for antifouling paint. As his-tory has proven, rather quickly in the latter case, neither ofthese claims was true.

The realities of gelcoat are: It provides a relatively sta-ble, aesthetically attractive finish that, under even the bestof circumstances, presents a modest barrier to water pen-etration.The ideal thickness for gelcoat is between 20 and30 mils (20 to 30 thousandths of an inch). Less than thisand it will not provide adequate coverage or quality of fin-ish, while gelcoats any thicker than 30 mils are prone tocracking.

Resin, the glue that binds the FRP structure together,may take several forms.Typically,even today,most boats aremanufactured using general-purpose polyester (PE) basedresin. Subcategories of this resin are orthophthalic andisophthalic,which simply refers to the acid from which theresins are manufactured.

Without delving too deeply into the chemistry of theseresins, the former is less expensive and less resistant to blis-

It is an old but particularlyappropriate expression

where barrier coatings areconcerned, “Pay a little nowor a lot later.” But what is abarrier coat, why is it some-times a necessary weaponin the war against osmoticblistering, and when shouldit and should it not beapplied?

As described in the maintext, the osmotic blister’smodus operandi involves theabsorption of water througha vessel’s skin coat and intothe laminate. For productionfiberglass-reinforced poly-ester (FRP) hulls, the skincoat is usually gelcoat; oncethought to be impervious towater penetration, navalarchitects, chemists andmany owners of vessels suf-fering from osmotic blisteringnow know otherwise.Although gelcoat formulationhas improved markedly overthe past decade, its useful-ness as a barrier coating ismarginal.

A true barrier coat willstem water penetration com-pletely. Unfortunately, nearlyevery plastic, regardless ofchemical makeup and densi-ty, will eventually absorbwater molecules. Thus, otherthan steel, aluminum andother metals, the best anFRP-boat owner can hope foris a barrier coating that willsignificantly slow down themigration of water mole-

Can barrier coats stop osmosis?

An epoxy barrier coatsprayed on a newly relami-

nated surface can stopfurther water permeation.

Circle No. 44 on the reader service card.

41www.oceannavigator.com

Coating Technology

tering or osmosis, while the latter is more expensive andmore resistant to osmotic attack.Because gelcoats are resin-based, they may also be orthophthalic- or isophthalic-based(ortho and iso in industry-speak), the latter sharing thesame attributes of blister resistance with iso-polyester gen-eral-purpose resin and thus the current preference. Nearlyall older boats were made using ortho resin and gelcoat.The demarcation varies, the switch occurring sometime inthe ’80s.

A relative newcomer to the resin scene is vinylester(VE), which has proven to be superior to its cousin, poly-

cules. This is either accom-plished through the chemicalmakeup of the product, suchas vinylester’s long, heavilyinterlocked molecular struc-ture, or by the addition ofsolids.

The use of vinylester lam-inates or skin coats, as wellas all-epoxy construction,has proven to be highlyeffective in preventing waterabsorption and the resultantosmosis. Unfortunately, thisdoesn’t help all the boatsthat are already in service —and those that will be built— using a more permeableresin such as polyester. Inthese cases, an aftermarketbarrier coat, which usuallyconsists of a high-solids-loaded epoxy, can be appliedas a prophylactic againstosmosis or blisters.

New polyester hulls arethe ideal candidates for bar-rier-coat application.Preventing water absorptionbefore it occurs is the pre-ferred and considerably lessexpensive approach toosmotic prevention. Although

the incidence of osmotic blis-tering in vinylester hulls isextremely rare, many ownersof these vessels often optfor the comparatively inex-pensive new boat-barrierapplication as an addedmeasure of security againstthe occurrence of blistering.In either case, it is generallyacknowledged that a barriercoat, particularly whenapplied to a vessel whennew, is a value enhancer.

Barrier coats can beapplied as part of a compre-hensive osmosis repairprocess; however, it is imper-ative that a barrier not beapplied over a bottom that is“wet” or continues to sufferfrom osmosis. This is simplya case of closing the prover-bial barn door after the horsehas escaped. Barrier coatinga wet laminate will neitherarrest the osmotic processnor prevent the occurrenceof new blisters. Laminatesmust be acceptably dry ornew in order to benefit froman osmotic barrier coat.

Steve C. D’Antonio

A boatbuilder applying gelcoat,the first step in fiberglass

laminate layup, to the inside of amold. While gelcoat was originally

considered to be impervious towater, it is now known that over

time some water can migratethrough gelcoat into the laminate.

For information circle No. 69 on the reader service card.



Coating Technology

ester, in nearly every way — including expense (it costsabout 15 percent more than PE). Vinylester resins areextremely tough and elastic while embodying excellentpermeability characteristics. Another valuable trait of VEresin is its compatibility with polyester resin.The two maybe used virtually interchangeably and in direct contactwith each other, with the same application equipment andcatalysts.As a result of these attributes,VE resin is now pre-ferred by high-quality hull manufactures either for theirentire laminate schedule or for the outer layers of the hull.

Finally, epoxy resin has gained favor in the boatbuilding

Barrier coatmanufacturers

Gougeon Brothers Inc.100 Patterson Ave.P.O. Box 908Bay City, MI 48707-0908989-684-7286www.gougeon.com

105 epoxy resin: 105resin is the base materialof the West System familyof products, on which all ofthe West System com-pounds are built. The resinis a clear, pale yellow, low-viscosity liquid epoxy resin.Formulated for use withWest System hardeners, itcan be cured in a widetemperature range to forma high-strength solid withexcellent moisture resist-ance.

Hawkeye Industries Inc.3050 Brookview Dr.Marietta, GA 30068 USA800-977-0060www.duratec1.com

Duratec: vinylesterprimer for one-off and pro-duction composite yachtpriming and osmosis andblister repairs. Formulatedfor below-waterline, topsideand deck applications.

International Paint Inc.2270 Morris Ave.Union, N.J. 07083908-686-1300www.yachtpaint.com

Interprotect barrier coat3000/3001: A high-solids,two-part epoxy coating,developed to protect oldand new fiberglass hullsfrom water absorption,which can lead to hull blis-tering

Interprotect barrier coat2000E/2001E:

Interprotect 2000E is thenewest development ofInterprotect 2000. It hasmany of the same out-standing properties butdoes not contain methyl-ene chloride. For this rea-son it is designated “E” asan environmentally pre-ferred product.

Interprotect barrier coat1000/1001: A high-buildepoxy primer for use whengelcoat is removed andfiberglass laminaterequires sealing.Interprotect 1000 is clear,and this enables the appli-cator to see that all thefibers are saturated andsealed.

VC Tar2: a two-compo-nent epoxy primer provid-ing osmosis protection toGRP boats and anticorro-sive protection to steeland alloy surfaces.

Interplastic Corp.1225 Willow Lake Blvd.St. Paul, MN 55110-5145651-481-6860www.interplastic.com

Low-VOC resins:CoRezyn CORVE8121LHseries of low hazardous airpollution (HAP) vinylesterresins. Regulatory-compli-ant marine vinylesterresins. They meet orexceed all U.S. standardsfor blister and fatigueresistance while meetingthe newest EnvironmentalProtection Agency stan-dards.

Gelcoats: Iso/NPG,isophthalic, low-VOC,vinylester, vinylester barrierand general-purpose gel-coat systems to protectfinished marine fiberglassparts.



43

Coating Technology

industry as a high-quality, extremely strong material that isalso virtually impermeable.Additionally, it is more environ-mentally friendly than ordinary poly and vinylesters, thanksto the low emissionsproduced during itscure cycle. Epoxy isby far the most ex-pensive of the avail-able boatbuildingresins, and it is ac-knowledged by man-ufacturers as some-what more difficultto work with. Satur-ating glass fabricwith epoxy is moretime consuming anddifficult than ordi-nary poly and vinyl-ester, and as a result,it is usually usedwhere high strengthand/or low emissions are required. It is worth repeatingthat epoxy laminates are among the strongest and most blis-ter-resistant structures. Epoxy is not readily compatiblewith poly and vinylester resins, nor is it compatible withglass fabrics whose binders and coupling agents (materialsthat hold the fabric together or promote resin bonding,

respectively) are formulated for poly and vinylester. Thisresin must only be used with epoxy-approved glass fabrics.

Glass-fabric reinforcement is available in scores of con-figurations, sizes and weights.The indi-vidual filaments used to weave differ-ent types of glass for FRP constructionare gossamer indeed, approximately1/10 the thickness of a human hair orabout 0.0002 inches. The principaltypes are chopped-strand mat, wovenroving and cloth.There are other moreexotic fabrics, such as knitted and biax-ials, but most boats, particularly those

that are suffering from blis-ters today, are built withthese three primary materi-als. They are all used in dif-ferent applications and fordifferent desired results.

From the standpoint ofosmotic blistering, the mostrelevant fabrics are the onesused just beneath the gelcoat,typically the chopped-strandmat and roving. Mat, which is

made up of short (about 2-inch), random lengths of glass fila-ments, is not as strong as the heavy rug-like weave of wovenroving. However, it is soft, sponge-like and absorbs resin

Spot-repairing blister domes by grindingand filling with epoxy or resin is strictlya cosmetic approach. The problemremains, and new blisters will continueto form.

A blister with dome removed,showing laminate structure.

Circle No. 63 on the reader service card. For information circle No. 13 on the reader service card.

44

Coating Technology

readily, and as such, it works well for bond-ing to other types of glass.Chopped-strandmat (CSM) comes in roll form and is heldtogether by a binding or sizing agent,which is designed to dissolve in resin,facilitating application of the mat intoirregularly shaped locations.

In order to work effectively with poly-ester or vinylester resin, glass fabrics mustbe treated with additives that keep the fab-ric bound together until it is wet-out withresin; these are known as sizing or bindingagents. Because of the short, random fibermakeup, binders are needed primarily forCSM fabrics.Additionally, glass is a relative-ly slick surface, so another agent,known asa coupler, is needed to allow the resin toget a grip on the filament.

Water, the universal solventThe hot-tub and spa industry faced the

osmotic blister problem in the ’60s. As itturns out, hot chlorinated water is theideal vehicle for promotion of osmotic blistering. Theirresponse was to get rid of gelcoat altogether, opting for anacrylic-sheet skin instead. Further study revealed thatacrylic is actually more permeable than good gelcoat that’sapplied in the proper thickness. The key to acrylic skin’ssuccess for the hot-tub folks was its lack of water-soluble

materials (WSMs).WSMs, as we’ll see, arethe primary villain in the fiberglass blistersaga.

In simplified form, the chemical pro-cesses that must occur for blistering areas follows. WSMs (the aforementionedbinders and couplers as well as thix-otropes, such as fumed silica, which pre-vent resin from being too thin and runny)must be present beneath a semiperme-able membrane — in this case, the gel-coat or skin coat.Water molecules, whichare comparatively small and slippery, findtheir way through the molecular gaps inthe gelcoat and FRP skin coat,where theyencounter the WSMs. It’s love at firstsight, and marriage ensues, but the off-spring are anything but cute.

As with any relationship, here’s whereit gets a bit tricky. Some compositeexperts believe that many of the WSMsaren’t present in the laminate immediate-

ly after the vessel goes into service. Rather, it’s only afterlong-term immersion that the process of hydrolysis, alsoknown as the Le Châtelier principle, begins to work on thelaminate, actually taking apart the resin matrix molecule bymolecule. The result is that water-soluble componentsbegin to appear in the laminate.

A hull peeler in action. This tool iscapable of removing gelcoat andlaminate with great precision.

For information circle No. 37 on the reader service card. Circle No. 77 on the reader service card.

45

The next pro-cess, the actualcause for the blis-ters themselves,then takes over.According toThomas J. Rock-ett Ph.D., a re-search professorat the Universityof Rhode Islandand co-author ofthe U.S. CoastGuard-fundedstudy The Causeof Boat HullBlisters, watermolecules enter the laminate via a process known as perme-ation.That in and of itself is not a problem as long as the waterdoesn’t react with anything on its journey through the lami-nate. Nearly all plastics, including FRP, are permeable to somedegree. The difficulty occurs when the water encounters areactionary agent, such as a WSM. Rockett describes theosmotic process as,“Water molecules can pass through this(semipermeable membrane, the gelcoat and laminating resin)layer, but the WSM molecules cannot (because they are largerthan water molecules). Since the outside water and the solu-tion are of different concentrations, water will permeate

through the gelcoat,in an attempt to dilute the droplet of solu-tion trapped in the laminate. During the process, more waterenters the droplet,causing it to expand and create pressure onthe surrounding hull material. It takes place whenever twosolutions of different concentrations are surrounded by asemipermeable membrane. When the pressure exceeds thedeformation point of the hull material, it begins to flow orcrack. This decreases pressure and allows more space forwater to be drawn into the solution.As the pressure grows, ablister forms on the surface.”

From this description, it is clear that the blister is thefinal step in the hydrolysis/osmosis problem. The afore-mentioned WSMs, coupled with permeability and theresultant susceptibility to hydrolysis of the resin matrix,appear to be the real culprits. Thus, one could concludethat the primary cause for osmotic hull blistering is theWSMs, although this borders on oversimplification,because the WSMs are one of several necessary ingredients.These necessary evils are the previous-ly mentioned couplers that allow resinto stick to glass filaments. Binders,particularly those applied asan emulsion, which were pop-

Coating Technology

Interlux’s Interprotect 1000 isa high-build epoxy primer usedfor sealing hull laminate after

gelcoat is removed.

The relaminationprocess can be

challenging, as it isessentially standard

fiberglass-boatbuilding, but from

the inside out.

Courtesy Interlux

For information circle No. 40 on the reader service card.Circle No. 4 on the reader service card.


Coating Technology

ular in the ’70s and early ’80s, that are used in some CSMand combination mat/woven/knitted-fiberglass cloth prod-ucts,have also been identified as having strong WSM poten-tial. Additionally, thickening agents or thixotropes such asfumed silica, which are added to resin to increase its vis-cosity (resin that’s too thin will simply run out of a lami-nate), are also water-soluble.

Adding insult to injury in the blister-formation story isthe recent release of research indicating that heavilystressed fiberglass laminates are more prone to osmosisthan their less-stressed counterparts. This makes sense,because stressed laminates tend to microfracture.(Sometimes the fractures aren’t so small. Ever see gelcoatcracks around on-deck hardware, cleats, chain plates, etc.?)These small fractures allow water to enter the laminatemore quickly than if it had to take the normal routethrough even a semipermeable gelcoat. Thus, it appearsthat where FRP strength is needed most — at the garboard,keel stub or adjacent to rudder attachments — is whereosmotic action may be most aggressive.

Studies also show that osmosis is accelerated considerablyin warmer water.Asa result, in Florida,for instance, osmot-ic blistering appearsto occur with great-er regularity than inMaine.However,be-ware of quick analy-ses from armchairchemists using an-ecdotal evidence.For example, theword on the streetis that boat A blisters badly, while boat B hardly ever gets blis-ters. If, however, boat A was manufactured in Florida and mostof the units were sold in the Southeast and,because of boat A’sdesign, it’s not used in colder climes, then boat A is certainlymore prone to blistering because of design as well as environ-mental factors. Boat B, on the other hand, having been madeusing similar materials and manufacturing processes, but inNew England, where it’s heavily marketed and, because of it’sdesign, is well suited for rougher, colder waters, may show farfewer examples of osmosis.

Add to this already complicated scenario a further wrin-kle. Evidence appears to suggest that prolonged immersionaccelerates hydrolysis and the osmotic process.Thus, boatsthat are used and stored in the northeast United States orGreat Lakes, where climate dictates seasonal hauling, areless likely to suffer from blisters than their tropical and sub-tropical brethren, regardless of laminate makeup. Simplyput, periodic hauling, which facilitates some drying of thegelcoat, tends to stave off or at least delay the onset ofosmosis.

There remains some disagreement on the subject of fre-quency of osmosis occurrence in fresh water vs. seawater.Some believe that osmosis occurs more quickly in the freshwater rather than salt water, while others believe theinverse.Those in the latter group point out that salts (not

West System 105 epoxy resin (shownwith hardener) can be cured in a widetemperature range to form a barrier coat.

Courtesy G

ougeon Brothers



47

Coating Technology

just sea salt or sodium chloride, but a vast array of ioniccompounds created during a combination of elements) pro-mote osmotic reactions. Science and history, however,appear to be on the side of accelerated osmosis occurringin fresh water because it is less dense than seawater andthus permeates semipermeable membranes with greaterease. Again, anecdotal evidence can be misleading. Whileosmotic blistering does occur in the Great Lakes, it is farless common on a per-capita basis than osmosis in thesoutheast United States.The Great Lakes freeze over, requir-ing boats to be hauled every winter, facilitating drying.

Resin manufacturers and laboratories that carry outosmosis-resistance tests nearly universally use hot, some-times boiling fresh water, in order to accelerate testing. Awet doormat, saturated with rainwater, left on a gelcoatedsurface will cause blistering, sometimes in a matter ofweeks. Experience shows, however, that osmosis doesoccur in both fresh water and seawater environments.

In the second part of this two-part series, we’ll explorethe details of what happens to an osmotically challengedhull, the chemistry of how it may be weakened, moisturetesting, osmosis repair and prevention. ■

Contributing Editor Steve C.D’Antonio is also the boat-yard manager of Zimmerman Marine in Mathews, Va.

Author’s note: In the interest of full disclosure, it is impor-tant to clarify that I am not a disinterested party where thesubject of fiberglass blisters is concerned. For the past 16years I have worked in the marine industry, the last sevenas the manager of a boatbuilding and repair yard that,among many other specialties, repairs osmotically blisteredboats. For the past 11 years, I have written and lecturedabout this and many other marine service and repair sub-jects.

Lest any reader draw the conclusion that my goal in writ-ing this article is self-serving, rest assured, the quantity ofrevenue derived from blister repair in the yard I manageconstitutes a small fraction of the overall business conduct-ed. Simply put, although I have supervised scores of theserepairs and no doubt will supervise many more in comingyears, osmosis repairs do not bear heavily on the profitabili-ty of my boatyard.

Although considered worthy of debate by some, all ofthe notions set forth in this text are well established withinrespected quarters of the professional boatbuilding, resin-manufacturing and blister-repair trades. I break no newground in what follows. My goal in writing this article iswholly educational, an attempt to assist the boatowner inmaking the best, most economically sound decisions aswell as preventing him or her from being misled by inaccu-rate information when contemplating a blister repair or thepurchase of a new and hopefully blister-resistant boat.

For information circle No. 5 on the reader service card.Circle No. 45 on the reader service card.

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50 OCEAN NAVIGATOR No. 138 May/June 2004

In the first of this two-part series onfiberglass blisters (see Fiberglassblisters and barrier coats Issue

136, March/April 2004), we detailedhow fiberglass boats are built, thematerials that are used and some ofthe causes of fiberglass blisters. Inthis, the second installment of theseries, we’ll explore the chemistry offiberglass that suffers from osmosis,as well as moisture analysis, repairstrategies and prevention.

For the most part, the causes ofosmotic blistering are well under-stood, by both the reader and theindustry as a whole. So one might ask,other than cosmetics,why is it a prob-lem? The answer to this question ismultipronged. Primarily, during theosmotic process, chemically acidiccompounds are created, such asacetic acid. Acetic acid attacks resin,leading to what the industry callsresin corrosion or fiber whiting. Thepurple, vinegar-smelling liquid (theLatin name for vinegar is acetum)that runs out of some burst osmoticblister is a result of water mixing withpolyvinyl acetate (PVA), which is used

as a coupling agent. When waterencounters the acetate component ofPVA, the byproduct is acetic acid.

When fiberglass laminates areremoved for blister or other repairs,these resin-starved areas are oftenmistaken for poorly wet-out glass fab-rics. Essentially, it’s easy to concludethat the vessel was built poorlybecause the laminating crew didn’ttake the time to ensure that the glassfilaments within the fabric were com-pletely saturated with resin. Thus,when the blister problem first beganto rear its ugly head, many in theindustry put two and two togetherand concluded that the cause ofosmotic blistering — poor wet-out —had been discovered. In fact, whilethis may be true in some cases, FRP(fiberglass-reinforced polyester orvinylester) laminates that are in anadvanced state of osmosis oftenexhibit large areas of fiber whiting,and experts now know this is aneffect of osmosis, not the cause.Theseareas of resin starvation or resin cor-rosion within an FRP laminate are notas strong as the day they left the mold

Story and photos by Steve C. D’Antonio

Bl is te r ana lys isand repairPart two of

our series onblistering in

fiberglass hullsand how it

can be fixed

or before they began to suffer theravages of osmosis. To what extentthey are weakened is subject to con-tinued debate.

An additional problem created byosmosis is the effect that waterabsorption has on polyester resin.Water is a plasticizer: When plastics(such as FRP) absorb water, theybecome pliable. This softening hassome effect on the fatigue resistanceof an FRP laminate. Under the rightcircumstances, severely saturatedlaminates may work-harden or crys-tallize at hard points, such as bulk-heads, stringers and keel stubs. Thequestion is, just how much will a wet,plasticized laminate flex comparedwith a dry laminate? Unfortunately,no one really knows for sure.There isa host of variables, and if the testswere done with current resins, itwould be meaningless for those olderresins that are of a different chemicalmakeup and have been in service formany years. All other theories aside,water absorption by FRP laminates isless than beneficial.

We know today that the majorityof osmosis problems originate fromresin and glass fabric additives — thewater-soluble materials (WSMs) —rather than insufficient wet-out ofthe fibers when the hull is laid up.Although poor wet-out can acceler-ate osmosis — each glass filamentthat is not saturated with resinbecomes a wick for water ingressinto the FRP laminate — it is of sec-ondary concern. Short glass fila-ments, such as those used in choppedstrand mat (CSM), tend to promoteosmosis; however, as mentioned, it isreally the emulsion binders found inthis material that cause the problem.The short, wispy strands, which tendto poke through cured resin, are sim-ply a vehicle for water molecules toreach the WSMs that lie within thelaminate.

This theory is borne out by thefact that chopper gun–applied chop— similar to CSM, but applied with agun rather than in rolls — is less like-ly to blister, once again citing theUniversity of Rhode Island study. In

A fiberglass peeler at work. Sometimesthe only solution to blisters is removal ofseveral layers of fiberglass from a hull.

51

spite of the fact that it, too, is madeup of short, wick-like fibers, it lacksthe binding agent found in roll mat.Because it is applied with a gun froma spool of material that passesthrough cutting or chopping blades,it requires no sizing to maintain itsstructure until laminated with resin.

It is ironic that chopper gun lami-nates — frequently used in produc-tion boatbuilding and often lookeddown upon as machine- rather thanhand-built laminates — while per-haps not as sturdy as hand lay-ups, areless likely to fall prey to osmosis.

Finally, it’s worth noting that sever-al factors, in addition to the poor wet-out of the skin coat, can act as acces-sories to osmosis. Less-than-careful or -ideal FRP boatbuilding practices go along way toward assisting the osmosisdemon. These include inattentivenessto the timing window when applyingthe skin coat over the gel coat in themold, as well as allowing fiberglassfabrics to become contaminated withmoisture, sawdust and other contami-nants before they are used in FRP con-struction.

Moisture testingConfirming the presence of water

in a blistered hull may seem unneces-sary; if the blisters are there, then it’sobvious the laminate is wet, right?Not necessarily.There are a few, albeitless common, causes of blistering.Blisters found in a vessel’s topside,well above the waterline, are usuallynot the result of osmotic action.Defective or improperly catalyzedresin may develop blisters.Additionally, FRP manufacturing toolsthat are malfunctioning may promoteosmotic blistering through localizedover-catalyzation.

Additionally, knowing how deeplyaffected the laminate is by the pres-ence of water is important for therepair process. If, for instance, osmo-sis has affected only the gel coat, thenattacking the problem any deeperthan that is a waste of effort andmoney. Conversely, applying a surfacerepair to a laminate that is sufferingfrom deep water saturation is apply-ing the proverbial Band-Aid to a gap-ing wound.

Moisture testing comes in twoforms, nondestructive and destruc-



tive.The former can be performed bya boatowner with a minimal invest-ment in tools or time. Begin by sand-ing the antifouling paint from a 1-square-foot area of the hull, exposingbare gel coat or whatever is beneaththe paint (sometimes it’s a barriercoat from a previous, oft-times failed,osmosis repair). Do this in at least

one, but preferably several areasbelow the waterline. It is important tonote that most antifouling paintseither retain water or contain metal,both of which will affect moisturetests, and thus, antifoulants must beremoved from the equation duringany moisture analysis.

Over this area, secure a layer ofrestaurant-grade clear plastic foodwrap, using all-weather masking tape,or an equivalent waterproof tape. Donot use ordinary masking tape orduct tape; it is not water resistant.Leave the test area for several days. If,upon your return, moisture is detect-ed on the inside of the plastic,chances are good that the laminatecontains water. How much water andhow deeply it has penetrated is any-one’s guess. This is what the cheaptest buys you, a yes or no answerrather than one of degrees.

The remaining two tests are semi-destructive and destructive. The firstinvolves a moisture meter, which —through capacitance and impedance

measurement, essentially using radiowaves — assesses the amount ofmoisture in a laminate. Technically, itis nondestructive in that it readsthrough a laminate to a depth ofapproximately a quarter inch.However, in order to assess where thewater-saturation wave ends, somelaminate disassembly — and thusdestruction — is necessary.

A valuable toolIn the hands of an experienced

professional, a moisture meter is anextremely useful and valuable tool inthe osmosis analysis process. In thehands of an inexperienced user, how-ever, the results this tool yields arevalueless at best and costly at worst.Most capacitance-type moisturemeters will, for example, show ablock of ice to be a dry substrate.Therefore, these tools should only beused on a hull that has not experi-enced freezing temperatures in thepast 48 hours. Metal objects, such asan imbedded strut, fastener or even atank on the inside of a hull, will false-

A moisture meter, used by a seasonedprofessional, is a valuable tool indetermining the extent of water saturationin fiberglass. Note, readings taken throughantifouling paint are not definitive.



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ly peg a moisture meter as if it hadbeen placed on an aquarium.

The moisture meter test, some-times known as a patch test, beginsthe same way as the plastic-wrap test,with removal of antifouling paint inan area or areas roughly 10 or 12inches square. The gel coat or othersubstrate is then tested. If the meterreads “dry,” then no further testing isnecessary. However, if the hull hasblisters, this is unlikely. Then, in thehands of a skilled operator, a grinderis used to remove the first layer ofsubstrate, gel coat or barrier, expos-ing the first FRP laminate.This is test-ed with the meter and the resultsrecorded. The process is repeated,removing successively deeper lami-nates, one laminate at a time, until anacceptably dry laminate is reached.

The final destructive test involvesdrilling a 1-inch hole in the hull in oneor more locations. (Because repair of ahole like this can be expensive,whether osmosis repairs are under-taken or not, a seacock can beinstalled to fill the gap.) This providesa 1-inch sample of the bottom that canbe sent to a laboratory that specializesin this type of analysis. The presenceof moisture can be confirmed, lami-nate ply by laminate ply.

On the occasions where I’vedecided to confirm the results of themoisture meter with a lab test, theresults have always been parallel.Thus, I have faith in the moisturemeter, provided it is in the hands of askilled professional.

What is considered dry as far as amoisture test is concerned? It’s amus-ing to hear even seasoned profes-sionals refer to osmotically saturatedhulls as reading 50, 70 or 100 percentwet. In reality, the numbers are ameasure of a relative scale. For themeter I use, a Tramex, one of thescales is 0 to 100, and anything overabout 5 is considered compromisedand thus too wet to barrier coat orlaminate over with new FRP. That 5,however, translates to 0.5 percentmoisture by weight. Anything belowthat is, of course, cause for celebra-tion. Hulls in the sub-5 category aredry and will, in all likelihood, fail todevelop blisters, although numbersabove 5 do not guarantee protection



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from future blistering. A thoroughlywater-soaked hull may contain 2 to 3percent moisture by weight, and a100 on the same Tramex scale indi-cates only about 1.75 percent mois-ture by weight. As a standard fieldcheck, moisture meters should be cal-ibrated on a vessel’s topside, wellabove the waterline. Under all but themost unusual circumstances, theseshould read dry.

The repair processSince the advent of the osmosis-

induced fiberglass hull blister, severalrepair strategies have emerged.Initially, there was a period of trialand error, particularly in the early1980s. Repair yards struggled, withgood intentions, to cure this ill thatwas plaguing what was now knownto be far from maintenance-free FRP.

Once the causes of osmotic blis-

tering were defined clearly — essen-tially the permeation of water intowhat was hitherto thought imperme-able — the natural repair progressionleads toward drying out these water-logged laminates. In the early days,this involved removal of the gel coat,in order to let the hull breathe, thenallowing Mother Nature to suck themoisture out by simple evaporation,the same way a puddle evaporateswhen the sun comes out.

Unfortunately, the drying methodrarely yielded a long-term repair, thenand now. Hulls that indicated dryaccording to the moisture meter wereplaced back in service only to rede-velop blisters. Despite refinements tothe drying technique, the applicationof infrared heat, dehumidifiers andvacuum pumps, the results were lessthan positive. Most boats that weredried out and coated with a properbarrier, usually a form of epoxy, suf-fered from blisters at some point inthe near future, sometimes as long asthree or four years or as quickly as a

The resultsof a patchtest arewritten on ahull. Thepercentsymbolsseen hereare notcorrect, asall moisturemeters readout on arelativescale.

For information circle No. 63 on the reader service card. For information circle No. 64 on the reader service card.

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few months, after being placed backinto service.

Why is the drying process so inef-fective? Because the root causes ofthe osmosis, WSMs, are not removedthrough evaporation. They remain,waiting for even the smallest amountof moisture with which to react,beginning the osmosis process onceagain.Additionally, drying, even if suc-cessful, does not address the resincorrosion and delamination that usu-ally accompanies osmotic blisters.Finally, some of the osmotic byprod-ucts — acetic acid and glycols —evaporate very slowly, if at all, undereven ordinary atmospheric condi-tions. The likelihood of these chemi-cals evaporating out of a dense sub-strate such as FRP is slim, indeed. Leftbehind, they continue to take theirtoll on the resin, particularly theacetic acid.

Getting a peelWhen it became evident to the

boatyards that were carrying out thedrying of osmotically sick vessels thatthis was not a long-term solution, thesearch for a cure began anew. Themethod that was eventually devel-oped and the one that is used exten-sively today, involved peeling off theaffected laminate and relaminatingwith the improved vinylester resin.

After a moisture analysis has beencarried out, it is possible to deter-mine the depth to which an afflictedlaminate needs to be peeled. The“wet” laminate is peeled off using aplaner-like device, whose depth ofpeel can be controlled precisely toincrements as small as 1/32 of aninch. Most electric peelers utilize vac-uum containment, and as a result,they are neat, clean and efficient.Other hydraulically powered peelersuse water for cooling and carryingaway the removed FRP slurry. Thisdetritus is often captured beneaththe vessel by mesh plastic sheeting.Because osmotically affected FRP isfar from inert, it should not beallowed to run directly onto theground or into nearby estuaries.

Vinylester (VE), the repair resin ofchoice,has proven to be virtually blis-ter proof over the past 15 years, inboth field use and in laboratory test-ing.The reason for this is vinylesters



Coating Technology




are nearly immune to hydrolysis, thedisassembly of the resin matrix as aresult of long-term exposure to water.In keeping with the theory that allplastics permeate, recent studiesshow that VE resin will absorb waterand plasticize, to some extent. But VEwill not suffer from hydrolysis. If theresin fails to hydrolyze, WSMs, a nec-essary ingredient for osmosis, neverbecome available, and blisters neverform.

The relaminating process using VEresin, after the hull has been peeled

and properly prepared,must incorporate a min-imum of two laminatelayers or 1/10 of aninch of laminate. Thisdepth of applied mate-rial ensures that an

appropriate exothermic reaction orheating takes place. This reaction isnecessary for the resin molecularchains to interlink properly. Ideally,the laminate that is removed shouldbe duplicated exactly, once again tothe previously stated minimums. AnyFRP material that is removed mustalways be replaced. Although it istempting to add a little more, chancesare good that the naval architect whodesigned your boat knew what hewas doing, and thus, his laminateschedule is appropriate and is worthy

of duplication. For the same reason,failing to replace some or all of theremoved material is forbidden.

VE resin has a higher tensilestrength and tensile elongation factorthan the original PE resin as well aspossessing excellent secondary bond-ing attributes. Provided the moistureanalysis, peel, preparation and lay-upare carried out properly, the relamina-tion should be quite strong andimmune to future blisters.

Epoxy barrier coatThe final step and what may be

termed the suspenders of a belt-and-suspenders approach is the applica-tion of an epoxy barrier coat over therelaminated hull. My preference is fora high-solids, epoxy-based, warrantedcoating that is backed by a reputablemanufacturer of marine products.Resist the temptation to use productsthat make incredible or fantasticclaims. Instead, go with a proven per-former who has a long-term trackrecord of standing behind their prod-uct.

The depth of a peeler’s cuton a single pass caninclude gel coat and one ortwo laminates. Deeper cutsrequire multiple passes.

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Coating Technology

Because osmotic blister correctionis not a do-it-yourself process, choosethe yard that undertakes your blisterrepair carefully. Using the methoddescribed here or a reasonable fac-simile, a repair yard should be pre-pared to offer a warranty against blis-ter reappearance for a minimum offive years, preferably 10.Ask for refer-ences, and talk to the owners of boatswho have had the treatment, bothrecently and in years past. Inspect at

least one finished product and insiston a written, fixed price quote ratherthan a verbal or written estimate.

As the old expression goes, anounce of prevention is worth apound of cure.The owner of any ves-sel that is manufactured from PE resinand is not currently suffering fromblisters or a saturated laminate,should strongly consider the applica-tion of the barrier coat describedhere. A few barrier coat manufactur-

ers offer excellentosmosis warranties oftheir own, providedthe product isapplied over a certi-fied dry laminate.Applying a barriercoat to a new vesselthat has not yet been

coated with antifouling paint is rela-tively inexpensive, while applying abarrier to a dry laminate that hasalready been coated with antifoulingpaint is a bit more time consuming,requiring the removal of the antifoul-ing paint. This is still very muchworthwhile and recommended forblister prevention.

Today, high-quality boat manufac-turers are building entire vesselsusing VE or epoxy resin, which yieldsnot only a blister-resistant structurebut one of superior strength as well,or they are skin coating with VE resin.

If you are shopping for a newboat, the value of a VE resin laminate,skin coat or all epoxy must beweighed carefully. Additionally, a no-blister hull warranty of no less thanfive years should be considered a pre-requisite. ■

Contributing Editor Steve C.D’Antonio is a freelance writer andthe boatyard manager of ZimmermanMarine in Mathews, Va.



The business end of afiberglass peeler. Theinterchangeable bladesmust be kept sharp forprecise cutting.

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fiberglass blisters and barrier coats · osmosis. unfortunately, this doesn’t help all the boats...

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