Blister analysis and repair

In the first of this two-part series on fiberglass blisters (see Fiberglass blisters and barrier coats Issue 136, March/April 2004), we detailed how fiberglass boats are built, the materials that are used and some of the causes of fiberglass blisters. In this, the second installment of the series, we’ll explore the chemistry of fiberglass that suffers from osmosis, as well as moisture analysis, repair strategies and prevention.

For the most part, the causes of osmotic blistering are well understood, by both the reader and the industry as a whole. So one might ask, other than cosmetics, why is it a problem? The answer to this question is multipronged. Primarily, during the osmotic process, chemically acidic compounds are created, such as acetic acid. Acetic acid attacks resin, leading to what the industry calls resin corrosion or fiber whiting. The purple, vinegar-smelling liquid (the Latin name for vinegar is acetum) that runs out of some burst osmotic blister is a result of water mixing with polyvinyl acetate (PVA), which is used as a coupling agent. When water encounters the acetate component of PVA, the byproduct is acetic acid.

When fiberglass laminates are removed for blister or other repairs, these resin-starved areas are often mistaken for poorly wet-out glass fabrics. Essentially, it’s easy to conclude that the vessel was built poorly because the laminating crew didn’t take the time to ensure that the glass filaments within the fabric were completely saturated with resin. Thus, when the blister problem first began to rear its ugly head, many in the industry put two and two together and concluded that the cause of osmotic blistering — poor wet-out — had been discovered. In fact, while this may be true in some cases, FRP (fiberglass-reinforced polyester or vinylester) laminates that are in an advanced state of osmosis often exhibit large areas of fiber whiting, and experts now know this is an effect of osmosis, not the cause. These areas of resin starvation or resin corrosion within an FRP laminate are not as strong as the day they left the mold or before they began to suffer the ravages of osmosis. To what extent they are weakened is subject to continued debate.

An additional problem created by osmosis is the effect that water absorption has on polyester resin. Water is a plasticizer: When plastics (such as FRP) absorb water, they become pliable. This softening has some effect on the fatigue resistance of an FRP laminate. Under the right circumstances, severely saturated laminates may work-harden or crystallize at hard points, such as bulkheads, stringers and keel stubs. The question is, just how much will a wet, plasticized laminate flex compared with a dry laminate? Unfortunately, no one really knows for sure. There is a host of variables, and if the tests were done with current resins, it would be meaningless for those older resins that are of a different chemical makeup and have been in service for many years. All other theories aside, water absorption by FRP laminates is less than beneficial.

We know today that the majority of osmosis problems originate from resin and glass fabric additives &mdash the water-soluble materials (WSMs) &mdash rather than insufficient wet-out of the fibers when the hull is laid up. Although poor wet-out can accelerate osmosis &mdash each glass filament that is not saturated with resin becomes a wick for water ingress into the FRP laminate &mdash it is of secondary concern. Short glass filaments, such as those used in chopped strand mat (CSM), tend to promote osmosis; however, as mentioned, it is really the emulsion binders found in this material that cause the problem. The short, wispy strands, which tend to poke through cured resin, are simply a vehicle for water molecules to reach the WSMs that lie within the laminate.

This theory is borne out by the fact that chopper gun-applied chop &mdash similar to CSM, but applied with a gun rather than in rolls &mdash is less likely to blister, once again citing the University of Rhode Island study. In spite of the fact that it, too, is made up of short, wick-like fibers, it lacks the binding agent found in roll mat. Because it is applied with a gun from a spool of material that passes through cutting or chopping blades, it requires no sizing to maintain its structure until laminated with resin.

It is ironic that chopper gun laminates &mdash frequently used in production boatbuilding and often looked down upon as machine- rather than hand-built laminates &mdash while perhaps not as sturdy as hand lay-ups, are less likely to fall prey to osmosis.

Finally, it’s worth noting that several factors, in addition to the poor wet-out of the skin coat, can act as accessories to osmosis. Less-than-careful or -ideal FRP boatbuilding practices go a long way toward assisting the osmosis demon. These include inattentiveness to the timing window when applying the skin coat over the gel coat in the mold, as well as allowing fiberglass fabrics to become contaminated with moisture, sawdust and other contaminants before they are used in FRP construction.

Moisture testing

Confirming the presence of water in a blistered hull may seem unnecessary; if the blisters are there, then it’s obvious the laminate is wet, right? Not necessarily. There are a few, albeit less common, causes of blistering. Blisters found in a vessel’s topside, well above the waterline, are usually not the result of osmotic action. Defective or improperly catalyzed resin may develop blisters. Additionally, FRP manufacturing tools that are malfunctioning may promote osmotic blistering through localized over-catalyzation.

Additionally, knowing how deeply affected the laminate is by the presence of water is important for the repair process. If, for instance, osmosis has affected only the gel coat, then attacking the problem any deeper than that is a waste of effort and money. Conversely, applying a surface repair to a laminate that is suffering from deep water saturation is applying the proverbial Band-Aid to a gaping wound.

Moisture testing comes in two forms, nondestructive and destructive. The former can be performed by a boatowner with a minimal investment in tools or time. Begin by sanding the antifouling paint from a 1-square-foot area of the hull, exposing bare gel coat or whatever is beneath the paint (sometimes it’s a barrier coat from a previous, oft-times failed, osmosis repair). Do this in at least one, but preferably several areas below the waterline. It is important to note that most antifouling paints either retain water or contain metal, both of which will affect moisture tests, and thus, antifoulants must be removed from the equation during any moisture analysis.

Over this area, secure a layer of restaurant-grade clear plastic food wrap, using all-weather masking tape, or an equivalent waterproof tape. Do not use ordinary masking tape or duct tape; it is not water resistant. Leave the test area for several days. If, upon your return, moisture is detected on the inside of the plastic, chances are good that the laminate contains water. How much water and how deeply it has penetrated is anyone’s guess. This is what the cheap test buys you, a yes or no answer rather than one of degrees.

The remaining two tests are semidestructive and destructive. The first involves a moisture meter, which &mdash through capacitance and impedance measurement, essentially using radio waves &mdash assesses the amount of moisture in a laminate. Technically, it is nondestructive in that it reads through a laminate to a depth of approximately a quarter inch. However, in order to assess where the water-saturation wave ends, some laminate disassembly &mdash and thus destruction &mdash is necessary.


A valuable tool

In the hands of an experienced professional, a moisture meter is an extremely useful and valuable tool in the osmosis analysis process. In the hands of an inexperienced user, however, the results this tool yields are valueless at best and costly at worst. Most capacitance-type moisture meters will, for example, show a block of ice to be a dry substrate. Therefore, these tools should only be used on a hull that has not experienced freezing temperatures in the past 48 hours. Metal objects, such as an imbedded strut, fastener or even a tank on the inside of a hull, will falsely peg a moisture meter as if it had been placed on an aquarium.

The moisture meter test, sometimes known as a patch test, begins the same way as the plastic-wrap test, with removal of antifouling paint in an area or areas roughly 10 or 12 inches square. The gel coat or other substrate is then tested. If the meter reads “dry,” then no further testing is necessary. However, if the hull has blisters, this is unlikely. Then, in the hands of a skilled operator, a grinder is used to remove the first layer of substrate, gel coat or barrier, exposing the first FRP laminate. This is tested with the meter and the results recorded. The process is repeated, removing successively deeper laminates, one laminate at a time, until an acceptably dry laminate is reached.

The final destructive test involves drilling a 1-inch hole in the hull in one or more locations. (Because repair of a hole like this can be expensive, whether osmosis repairs are undertaken or not, a seacock can be installed to fill the gap.) This provides a 1-inch sample of the bottom that can be sent to a laboratory that specializes in this type of analysis. The presence of moisture can be confirmed, laminate ply by laminate ply.

On the occasions where I’ve decided to confirm the results of the moisture meter with a lab test, the results have always been parallel. Thus, I have faith in the moisture meter, provided it is in the hands of a skilled professional.

What is considered dry as far as a moisture test is concerned? It’s amusing to hear even seasoned professionals refer to osmotically saturated hulls as reading 50, 70 or 100 percent wet. In reality, the numbers are a measure of a relative scale. For the meter I use, a Tramex, one of the scales is 0 to 100, and anything over about 5 is considered compromised and thus too wet to barrier coat or laminate over with new FRP. That 5, however, translates to 0.5 percent moisture by weight. Anything below that is, of course, cause for celebration. Hulls in the sub-5 category are dry and will, in all likelihood, fail to develop blisters, although numbers above 5 do not guarantee protection from future blistering. A thoroughly water-soaked hull may contain 2 to 3 percent moisture by weight, and a 100 on the same Tramex scale indicates only about 1.75 percent moisture by weight. As a standard field check, moisture meters should be calibrated on a vessel’s topside, well above the waterline. Under all but the most unusual circumstances, these should read dry.

The repair process

Since the advent of the osmosis-induced fiberglass hull blister, several repair strategies have emerged. Initially, there was a period of trial and error, particularly in the early 1980s. Repair yards struggled, with good intentions, to cure this ill that was plaguing what was now known to be far from maintenance-free FRP.

Once the causes of osmotic blistering were defined clearly &mdash essentially the permeation of water into what was hitherto thought impermeable &mdash the natural repair progression leads toward drying out these waterlogged laminates. In the early days, this involved removal of the gel coat, in order to let the hull breathe, then allowing Mother Nature to suck the moisture out by simple evaporation, the same way a puddle evaporates when the sun comes out.

Unfortunately, the drying method rarely yielded a long-term repair, then and now. Hulls that indicated dry according to the moisture meter were placed back in service only to redevelop blisters. Despite refinements to the drying technique, the application of infrared heat, dehumidifiers and vacuum pumps, the results were less than positive. Most boats that were dried out and coated with a proper barrier, usually a form of epoxy, suffered from blisters at some point in the near future, sometimes as long as three or four years or as quickly as a few months, after being placed back into service.

Why is the drying process so ineffective? Because the root causes of the osmosis, WSMs, are not removed through evaporation. They remain, waiting for even the smallest amount of moisture with which to react, beginning the osmosis process once again. Additionally, drying, even if successful, does not address the resin corrosion and delamination that usually accompanies osmotic blisters. Finally, some of the osmotic byproducts &mdash acetic acid and glycols &mdash evaporate very slowly, if at all, under even ordinary atmospheric conditions. The likelihood of these chemicals evaporating out of a dense substrate such as FRP is slim, indeed. Left behind, they continue to take their toll on the resin, particularly the acetic acid.

Getting a peel

When it became evident to the boatyards that were carrying out the drying of osmotically sick vessels that this was not a long-term solution, the search for a cure began anew. The method that was eventually developed and the one that is used extensively today, involved peeling off the affected laminate and relaminating with the improved vinylester resin.

After a moisture analysis has been carried out, it is possible to determine the depth to which an afflicted laminate needs to be peeled. The “wet” laminate is peeled off using a planer-like device, whose depth of peel can be controlled precisely to increments as small as 1/32 of an inch. Most electric peelers utilize vacuum containment, and as a result, they are neat, clean and efficient. Other hydraulically powered peelers use water for cooling and carrying away the removed FRP slurry. This detritus is often captured beneath the vessel by mesh plastic sheeting. Because osmotically affected FRP is far from inert, it should not be allowed to run directly onto the ground or into nearby estuaries.

Vinylester (VE), the repair resin of choice, has proven to be virtually blister proof over the past 15 years, in both field use and in laboratory testing. The reason for this is vinylesters are nearly immune to hydrolysis, the disassembly of the resin matrix as a result of long-term exposure to water. In keeping with the theory that all plastics permeate, recent studies show that VE resin will absorb water and plasticize, to some extent. But VE will not suffer from hydrolysis. If the resin fails to hydrolyze, WSMs, a necessary ingredient for osmosis, never become available, and blisters never form.

The relaminating process using VE resin, after the hull has been peeled and properly prepared, must incorporate a minimum of two laminate layers or 1/10 of an inch of laminate. This depth of applied material ensures that an appropriate exothermic reaction or heating takes place. This reaction is necessary for the resin molecular chains to interlink properly. Ideally, the laminate that is removed should be duplicated exactly, once again to the previously stated minimums. Any FRP material that is removed must always be replaced. Although it is tempting to add a little more, chances are good that the naval architect who designed your boat knew what he was doing, and thus, his laminate schedule is appropriate and is worthy of duplication. For the same reason, failing to replace some or all of the removed material is forbidden.

VE resin has a higher tensile strength and tensile elongation factor than the original PE resin as well as possessing excellent secondary bonding attributes. Provided the moisture analysis, peel, preparation and lay-up are carried out properly, the relamination should be quite strong and immune to future blisters.

Epoxy barrier coat

The final step and what may be termed the suspenders of a belt-and-suspenders approach is the application of an epoxy barrier coat over the relaminated hull. My preference is for a high-solids, epoxy-based, warranted coating that is backed by a reputable manufacturer of marine products. Resist the temptation to use products that make incredible or fantastic claims. Instead, go with a proven performer who has a long-term track record of standing behind their product.

Because osmotic blister correction is not a do-it-yourself process, choose the yard that undertakes your blister repair carefully. Using the method described here or a reasonable facsimile, a repair yard should be prepared to offer a warranty against blister reappearance for a minimum of five years, preferably 10. Ask for references, and talk to the owners of boats who have had the treatment, both recently and in years past. Inspect at least one finished product and insist on a written, fixed price quote rather than a verbal or written estimate.

As the old expression goes, an ounce of prevention is worth a pound of cure. The owner of any vessel that is manufactured from PE resin and is not currently suffering from blisters or a saturated laminate, should strongly consider the application of the barrier coat described here. A few barrier coat manufacturers offer excellent osmosis warranties of their own, provided the product is applied over a certified dry laminate. Applying a barrier coat to a new vessel that has not yet been coated with antifouling paint is relatively inexpensive, while applying a barrier to a dry laminate that has already been coated with antifouling paint is a bit more time consuming, requiring the removal of the antifouling paint. This is still very much worthwhile and recommended for blister prevention.

Today, high-quality boat manufacturers are building entire vessels using VE or epoxy resin, which yields not only a blister-resistant structure but one of superior strength as well, or they are skin coating with VE resin.

If you are shopping for a new boat, the value of a VE resin laminate, skin coat or all epoxy must be weighed carefully. Additionally, a no-blister hull warranty of no less than five years should be considered a prerequisite.

Contributing Editor Steve C. D’Antonio is a freelance writer and the boatyard manager of Zimmerman Marine in Mathews, Va.

By Ocean Navigator