Few materials rival fiberglass. It has several advantages over steel. For example, low-volume parts made from it cost far less than steel ones. It resists more chemicals, including an abundant one that causes steel to whither away into brown dust: oxygen. Size being equal, properly made fiberglass can be several times stronger yet still lighter than steel. In fact, it won't even dent.
No, when fiberglass does fail, it cracks. In fact, if hit hard enough, fiberglass breaks into pieces.
Steel snobs rejoice at that last part, but little do they know that's where fiberglass really begins to shine. A seasoned veteran with a shop full of specialized tools might not be able to justify repairing a flat-smashed metal fender, but a rank amateur with hardly more than simple hand tools can put a fiberglass one back together from dozens of shattered pieces ... sometimes in one afternoon.
To learn how to fix it properly, we have to understand how fiberglass works in the first place. Depending on the construction technique, fiberglass resembles either a concrete slab or a sheet of plywood.
A slab owes its strength to the steel reinforcing bar, or rebar, embedded in it; the concrete is merely the glue that binds the bars and offers a conveniently smooth surface. Trade the rebar for a lot more glass strands, the concrete for considerably less plastic resin, and scale it down a whole bunch, and you've basically got a fiberglass part made with a chopper gun.
Wood plies bound together by glue create a panel far stronger than solid wood of comparable size. This construction forces individual plies to bear loads in tension or compression, the directions where most materials are strongest. We refer to fiberglass parts made by a similar bonded-ply technique as hand-laid or hand-laminated.
While more labor intensive, hand-lamination has serious benefits. First, it doesn't require any special equipment like chopper guns, so it's a real asset to occasional users. But most importantly, hand-lamination makes far stronger parts than any chopper gun could ever aspire to. It's because fiberglass owes its strength almost exclusively to glass fibers and hand-lamination achieves a far better glass-to-resin ratio than a gun can. Remember that the resin exists only to bind the fibers. It's comparatively brittle, and any more than necessary makes parts weaker and heavier.
Like an iceberg, fiberglass damage is often a lot bigger under the surface. Start every re
This distinction is important for more than just selecting fiberglass parts; the hand-lamination technique is the backbone of most fiberglass repairs. Rather than merely joining broken materials at the point of damage as we do when welding metal, we literally grind away the damage and replace it with new material. By grinding the damaged panels in a particular manner, fiberglass repairs achieve great surface-area contact, which is essential to ply construction technique. What's more, a properly made repair is every bit as strong as the remainder of the panel. In some cases-particularly with chopper gun-made parts-repairs made by this technique can be stronger than the existing panel. But best of all, any enthusiast with a few very common tools and a good supplier can repair fiberglass with the same kind of quality and reliability as a seasoned veteran can offer.
Though we can't anticipate every type of damage, this method applies to 99 percent of all fiberglass repairs. In fact, this information applies to things like chopping fiberglass tops and grafting together two panels. Only the person doing the chopping is creating the damage. The repairs after the modifications remain largely the same.
While we don't think you'll intentionally create damage just to get the opportunity to try out this technique, merely knowing how to do it certainly eliminates a lot of anxiety. At the very least you'll rest easy knowing that strong and reliable fiberglass repairs are easier than you thought.
This panel's damage didn't extend very far beyond what the surface indicated. It did, howe
This exploded fiberglass panel diagram illustrates this repair technique. Note how the exi
Sketch a border as far as reasonably possible around the damage. The high crown and comple
Base your repair on mat since it more readily conforms to surface irregularities. Cut each
I buried the cloth with more mat to prevent its pattern from "printing" through the finish
Feather-edging the crack usually creates a gap, especially if the crack is nasty. No probl
Tip: Polyester resin sticks to Mylar like Kirstie Alley sticks to a diet, so I clad the ch
Fiberglass repair is messy but cleanliness is the cornerstone of long-lasting repairs. Pla
Before mixing a batch, refer to the information about catalyzing resin.
Tip: Plastics shop
Dip a small disposable paint brush roughly a third into the resin then gently tap the brus
Position the second layer, only instead of brushing on more resin, roll the layer to make
Continue the laminating technique alternating between the roller and brush to saturate eac
Finally apply the frayed mat layers. Technically I'm cheating since I'm applying resin bef
The roller expels bubbles and excess resin. It takes a deft technique: too little pressure
For whatever reason I overlooked this small nearby hole (finger). Holes require the same b
Fiberglass rewards creativity. The compound shape of this repair means I can't use cardboa
Foam shapes incredibly easily with sandpaper and quickly with a grinder. I roughed this on
Tip: Use the panel profile to shape the foam. I held a piece of 60-grit paper to the panel
After about 30 seemingly eternal seconds of arduous labor, the foam backer matched the pan
There's one problem with Styrofoam: resin melts it. Once again, Mylar packing tape to the
As we did with the crack, we ground the panel in a gentle taper and knife-edged the hole.
Tip: You'll end up with lots of scrap mat after trimming for a few jobs. Don't throw it aw
The scraps are valuable because individual laminates don't need to cover the entire hole i
Gauge the repair's consistency by both eye and roller feel whenever using random pieces (a
"Look ma, no hands" might have been bragging rights when you were a kid, however, fibergla
A Word On Safety
Working with fiberglass-especially grinding the stuff-generates zillions of near-microscopic shards of glass. And those shards itch like hell when they work their way into your skin's pores. The stories of itchy armpits and crotches make good bench racing. Only there's nothing funny about those shards making their way into your lungs, which underscores the importance of quality protective gear. In that case, those shards can lacerate the sacs in your lungs causing an irreversible condition called Pulmonary Fibrosis. It reduces lung capacity and generally makes for a miserable and crippled life. Something as simple as a dust mask can go a long way to preserve quality of life. Better yet, use a painter's respirator. Resin and acetone fumes might lend a stoney feeling to the day, but prolonged exposure can make you long-term dumb. Oh yeah, make sure your work area gets good ventilation. No respirator, no matter how powerful, will protect you in a fume-saturated workspace.
Resins and solvents also wick through skin like fruit punch through a paper towel and melt kidneys and livers as they leave the building. Latex gloves work great, and considering that constant contact with chemicals is inevitable you're going to whip through gloves the way a jilted teenager goes through tissues.
Not even an eyeball hardly stands a chance. A chunk of fiberglass kicked up by a grinding wheel cuts like a flying razor blade. A single drop of the potent oxidizer used to catalyze resin splashed in the eye can permanently blind. Goggles are a good starting point, but don't underestimate the comfort and peace of mind a full face shield offers. Prescription eyeglass wearers take special note.
There are also optional protective measures. Long pants taped to ankles and long-sleeve shirts taped to wrists may make you sweat, but you'll appreciate not itching at the end of the day. Tool suppliers sell lotions that seal pores, preventing glass fibers from lodging into pores. Hats and bandannas keep your hair clean, and so on.
Make no mistake: Fiberglass is relatively safe when handled properly. It's also capable of inflicting unspeakable pain. Treat it (and yourself) with care.
Nearly any polyester resin from a good supply store will work, but hold out for Iso resin if possible. Avoid surfacing resins for these types of repairs; the paraffin wax suspended in them lets the surface dry more completely but it wreaks havoc with paint and future laminations unless you sand the dried surface completely.
Like paint, fiberglass resin comes in several forms: polyester, vinylester, and several grades of epoxy just to name a few. While some of these formulations offer incredible strength and durability, J.B. Donaldson recommends the standby, polyester resin. "Yeah, the other stuff can be a lot stronger, but it can also be tremendously difficult to work with," he cautioned. He noted that alternative resins often require very specific measurements (mostly by weight), mixing procedures, and temperatures to achieve their full strength.
"They can also make future work really difficult," he added. While epoxy will stick to polyester resin (epoxy primer-sealers over raw fiberglass and polyester-based fillers), the inverse isn't necessarily true. "You can't just go over epoxy with polyester," he noted. "It just doesn't stick, which makes future repairs on a part that much more difficult."
Like even the simplest enamel, even polyester resins come in several forms. Before we proceed, though, let us offer a word of caution: avoid resin sold at chain stores. Inexpensive resin-often brown, coagulated stuff-pours terribly, mixes terribly, and wets out materials terribly. It also makes brittle and weak parts when the stuff hardens-if at all in some cases.
Instead, find a plastics supply or a very good boat house, whether locally or by mail order. J.B. says to specify Isophthalic resin, often referred to as Iso or tooling resin. He noted that while Orthophthalic resins are adequate, they're not nearly as strong as Iso resins, especially for the very modest savings. The price difference sometimes works in favor of the good stuff. My last batch of Iso resin, for example, costs less than the green can of stinky brown death at the department store and a third of the price of the stuff at the local parts house.
Supply houses offer something else that no box store can: service. A blue-vested associate can hardly tell you what aisle to find resin much less how to use it. Supply houses often have intimate knowledge of their products, whether firsthand or through their customers' experiences, and can suggest other supplies and materials to make your work that much better or easier.
Dedicated shops also offer fresh materials, a big deal since even a virgin can of polyester resin definitely has a shelf life.
How To Catalyze Resin
Catalyst content is key to a proper cure rate and strength. Luckily most catalyst bottles feature dropper tops for fairly precise measurements. Refer to the resin-to-catalyst chart to get into the ballpark.
Resin cross-links or polymerizes in its own sweet time but heat can speed up the process. Large manufacturers use giant ovens, but very small doses of a really potent oxidizer called methyl ethyl ketone peroxide, or MEKP, causes a chain reaction that generates enough localized heat to cross-link the resin.
There's a catch, though: this resin-to-catalyst ratio is very sensitive. Too little and the resin may not cure, too much will cause the resin to craze in an array of microscopic cracks that weaken the structure. Way too much, in fact, will start a fire. Making matters more complicated, temperature plays a similar albeit less-dramatic role in this chain reaction.
Chemical manufacturers rate their formulas to work best at 70 degrees F, or 21 degrees C, with 1 percent of their weight in catalyst. Neither figure is etched in stone. In fact, they're inversely related: as one goes up, the other goes down. For example, resin requires less catalyst to polymerize in hotter ambient temperatures and more catalyst to polymerize in cooler temperatures.
Bear in mind this fudging has limitations. Less than 0.75 percent catalyst probably won't polymerize resin; more than 1.5 percent will cause damage or a fire. This might be enough to extend your working range as low as 60 degrees F to as high as 95. "But don't try to go too far," J.B. warns. "Heat up your shop if it's colder than about 60 degrees," he instructs. My local supplier suggested building boxes to contain projects and warming the space inside with a heat lamp. Believe it or not, but it worked for me on a 40-degree day.
As tedious as this weight/volume ratio sounds, it's not tedious in application. For example, eight drops of MEKP works out to 1 percent of an ounce of resin. Vendors know this and usually package small amounts of MEKP in squeeze bottles that conveniently dispense in drops. Use this chart as a working range. Remember to always use at least 0.75 percent catalyst and never exceed 1.5.
We purposefully chose relatively small amounts in the chart to show the importance of mixing only what you can use within a 20-minute period. Even then, you'll find it fairly challenging to properly use 8 ounces of catalyzed resin before it starts to set up in the cup. While manufacturers often mix by gallons, bear in mind that they use tooling and/or production methods that consume the materials far quicker than a bunch of us hobbyists can. Proper technique takes time.
One more thing bears mentioning when talking resin: mixing cup size. It sounds strange at first, but a given amount of resin will polymerize far quicker in a smaller cup than it will in a larger cup. It goes back to the heat: Concentrating catalyzed resin in a smaller container also concentrates the heat it generates. To get the longest pot life from your resin, mix it in wide, shallow cups. By itself that may double resin's pot life without affecting its cure rate.