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...
Like an iceberg, fiberglass damage is often a lot bigger under the surface. Start every repair with some exploratory "surgery." Grind away the paint and/or gel coat several inches beyond the crack but don't grind too far into the fiberglass itself ... yet.
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...
This panel's damage didn't extend very far beyond what the surface indicated. It did, however, expose a gigantic air bubble nearby (arrow). Technically we should fix that while we're here, but we'll ignore it for now.
This exploded fiberglass panel...
This exploded fiberglass panel diagram illustrates this repair technique. Note how the existing panel tapers toward the damage. See how the new layers overlap in increasing size to fill the depression? The great surface area between the surface and repair is essential to a long-lasting repair; anything less will pull away.
Sketch a border as far as...
Sketch a border as far as reasonably possible around the damage. The high crown and complex shape make this panel strong, so I gave the repair area a bit more than an inch on each side of the damage. Carefully grind the panel from full thickness at the line to a sharp edge at the damage.
Base your repair on mat since...
Base your repair on mat since it more readily conforms to surface irregularities. Cut each successive layer larger than the prior so they overlap to match the taper ground into the panel. I use 3/4-ounce mat for smaller repairs with complex shapes but a 2-ounce mat split into two thinner plies works well. Note that I used stronger 6-ounce cloth for the fifth layer.
I buried the cloth with more...
I buried the cloth with more mat to prevent its pattern from "printing" through the finish. I frayed the last layers' edges since hard-cut mat edges can print through just the same as the cloth pattern.
Feather-edging the crack usually...
Feather-edging the crack usually creates a gap, especially if the crack is nasty. No problem though; just make a backing. Chip board works fine on flats to simple curves but we'll show another option later.
Tip: Polyester resin sticks...
Tip: Polyester resin sticks to Mylar like Kirstie Alley sticks to a diet, so I clad the chip board with plain ol' packing tape to prevent the chip board from becoming one with the lamination. Using the same stuff, I taped the chip-board backing to the panel.
Fiberglass repair is messy...
Fiberglass repair is messy but cleanliness is the cornerstone of long-lasting repairs. Plain ol' acetone removes dirt and grease and strips the uppermost film of cured resin giving the new resin a perfect bonding surface.
Before mixing a batch, refer...
Before mixing a batch, refer to the information about catalyzing resin.
Tip: Plastics shops and marine houses sell very good mixing hardware like these smaller graduated cups. Use them to transfer resin from the can to your larger mixing cup (background). Use plastic or plain paper cups since wax-coated cups wreck laminations.