Fiberglass 101: A Boater’s Guide to Cloth, Mat & Resin

Whole how-to books could be written about fiberglass boat building and repair. In fact, many have been. Part science and part art, fiberglass fabrication is something that skilled fabricators spend their whole lives perfecting. Before you start learning the “how,” though, it’s important to understand the “what.” 

Because fiberglass fabrication terminology is so full of arcane words and acronyms, most boaters don’t have more than the most basic understanding of how fiberglass boats are made. Let’s go over the basic terminology and concepts behind marine fiberglass construction.

To turn it into the tough, stiff material that boat hulls are made from, it must be saturated with a polymer resin and allowed to cure.

The result is called a composite, which simply means something made from a combination of materials — in this case, glass fibers and resin. Technically, it’s no longer just fiberglass; instead, it’s fiber-reinforced plastic (FRP) or glass-reinforced plastic (GRP).

Like cotton fibers, glass fibers can be made into a variety of different textiles with different properties.

Fiberglass mat — commonly called chopped strand mat or “CSM” — is made of small bundles of fibers chopped into short lengths and oriented randomly. Because CSM isn’t woven or stitched, the fibers have to be stuck together with a “binder” so they form a sheet.

One way to think of CSM is to imagine thousands of two-inch pieces of fishing line dropped at random onto a flat surface and then coated with an adhesive spray to stick them all together into a mat.

Woven roving is still another kind of fiberglass cloth. Woven roving is made from bundles of fibers — known as “rovings” — that are woven under and over each other at 90-degree angles to form a coarse cloth. The result is thick, solid cloth that looks something like a checkerboard pattern. 

There are other kinds of fiberglass cloth, including triaxial and even quadaxial stitched fiberglass made by stacking three or four non-woven layers in different orientations. CSM, biaxial and woven are the most commonly used in recreational boat building though.

Typically, fiberglass cloth is described by a weight in ounces, e.g., 17 oz. biaxial or 1.5 oz. chopped strand mat. The number refers to the approximate weight of a single square yard of the fabric. The higher the weight, the thicker or denser the cloth. Keep in mind that this is not apples-to-apples. For example, 18 oz. biaxial fiberglass will yield a stiffer and stronger laminate than 18 oz. woven.

Fiberglass textiles have very different properties from one another that dictate what they’re used for in boat building or repair.

First, dry CSM (before it is wetted with resin) is lightweight and flexible. That makes it easy to work with. It’s also inexpensive. Second, CSM holds a lot of resin, which makes it an excellent bonding layer. In fact, bonding layers together is one of the CSM’s primary uses. Third, CSM tends to yield a smoother finished surface than knit or woven cloth.

As a result of all three, CSM is commonly used for the first layer of fiberglass behind the gelcoat in boat hulls and boat parts. With other kinds of fiberglass, the pattern of the glass can show through the gelcoat.

This is called “print through.” CSM also conforms easily to sharp angles and provides a strong bond between the gelcoat and the next layer of fiberglass.

In biaxial fiberglass, the fibers in the two layers are aligned at 90 degree angles to each other. In so-called “0/90” biaxial, one layer runs directly across the roll of fabric and the other runs along the roll. In 45/45 biaxial, one layer is at 45 degrees and the other is at negative 45 degrees, forming something like an X pattern.

For fiberglass laid along a seam or angle, 45/45 is normally used. With 0/90, the layer of fibers running along the length of seam or angle provides little reinforcement. But with 45/45, both layers cross the seam or angle and add strength.

Because of its thickness, though, woven roving is commonly used in fairly flat applications to quickly build up the thickness of the laminate, usually with a layer of CSM between the layers of woven roving to achieve better bonding.

Another popular and versatile option is 1808, which normally comprises 18 oz. 0-90 biaxial and 3/4 oz. CSM. Lighter versions, like 1208, are also available.

A single layer of fiberglass and resin is rarely used in boat building. Instead, boats and boat parts are made with laminates — several layers of fiberglass, resin, and often other materials sandwiched together into a single cohesive panel.

The sides of a typical fiberglass boat hull might consist of a layer of gelcoat on the outside, then a layer of chopped strand mat, then one or more layers of knitted and/or woven cloth, then a piece of coring material, then more layers of cloth. Coring is used to make laminate panels lighter and stronger. It can be almost anything, from plywood to lightweight honeycomb panels to high-density closed-cell foam.

The exact combination of fiberglass layers and coring is different in different parts of a boat. For example, a boat bottom might be made with many built-up layers of fiberglass and no coring for maximum impact resistance, while a fiberglass hardtop might be made with honeycomb coring and far less glass for weight savings. Transoms and stringers are frequently cored with high-density reinforced foam like Coosa board.

Again, the language of fiberglass fabrication can be almost incomprehensible to the average boat owner. But here are a few definitions that might help demystify things.

Layup schedule — A layup schedule has nothing to do with time. Instead, it’s a “recipe” for the layers in a fiberglass part, detailing what kinds of cloth are laid down, what kinds of resin are used, and what, if any, coring is used.

Wet Out — The exact combination of fiberglass layers and coring is different in different parts of a boat. For example, a boat bottom might be made with many built-up layers of fiberglass and no coring for maximum impact resistance, while a fiberglass hardtop might be made with honeycomb coring and far less glass for weight savings. Transoms and stringers are frequently cored with high-density reinforced foam like Coosa board.

Vacuum Bagging — Vacuum bagging uses vacuum pressure to draw excess resin out of a laminate. The fiberglass and resin are laid as usual, then, before curing, the entire part is wrapped in plastic film and a vacuum is applied, pulling extra resin out and reducing weight without compromising strength.

Resin Infusion — Also called vacuum infusion and vacuum-assisted resin infusion, this is the process of using vacuum pressure to suck resin into fiberglass cloth. Basically, layers of cloth are “stacked” and smoothed without resin, then the whole hull or part is wrapped in air-tight plastic. A vacuum is applied, pulling resin through hoses and into the laminate. Done well, infusion can save not only weight but also money, since boat builders can measure the optimal amount of resin and reduce waste.

Hand Laid — Hand laid describes fiberglass that has been wet out and laminated by hand using a variety of metal rollers and plastic squeegees. Workers position and wet out layers of cloth one by one. Hand laid fiberglass tends to be heavier than infused fiberglass, since more resin is usually used.

Polyester — Polyester resin is the basic resin used in fiberglass boat building. It’s inexpensive, relatively easy to work with and has good bonding qualities.

Vinylester — Superior to polyester resin in most aspects, vinylester resin is stronger, less prone to cracking and fatigue, and more water resistant. It’s also more expensive and more demanding to work with.

Epoxy — Used as a resin, epoxy is stronger and more rigid than vinylester or polyester and a much better adhesive than either. It’s also waterproof. On the other hand, it’s more expensive even than vinylester and more challenging to work with.

E-Glass — The most commonly used fiberglass, E-glass is relatively inexpensive and suitable for a wide range of boat building applications.

S-Glass — Considerably more expensive than E-glass but also stronger, stiffer and more tolerant of deformation, S-glass is sometimes used in high-performance marine applications.

Primary Bonding — Primary bonding is bonding between two uncured layers of fiberglass and resin. Basically, a new layer of glass is put in place and wet out before the last one cures. The two layers essentially become one.

Secondary Bonding — Secondary bonding is bonding between a cured and an uncured layer. For example, glassing a stringer to the bottom of a cured hull depends on secondary bonding. Some resins have better secondary bonding qualities than others.