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Back to Basics with Fiberglass


Who doesn't love the smell of resin and that wonderful feeling of fiberglass on your skin?


Ah, that sweet temptress... Nothing quite like it!


While it's not always pleasant working with fiberglass, sometimes it is a necessity. With this handy guide, hopefully it won't be too daunting! There are hundreds of different types of fiberglass, from roving to stitched, to chop strand, all in different orientations, weaves and thicknesses, but to simplify matters we will only address three main types in broad terms: Chopped Strand Mat, Woven Roving, and Stitched Bi-axial.



Chopped Strand Mat

Affectionately known as the devil's armpit hair or CSM. This mat is made up of short strands of glass that are randomly scattered and bound to create a mat which can then be wetted out with resin. It is widely used in mould making and the production of low cost GRP mouldings such as panels and small boat hulls. The strands are bound with emulsion that is suitable for use with polyester and vinylester resins which will quickly break down the emulsion binder so that the mat can easily be laminated by brush or roller to follow the contours of the mould. These strands also make it an ideal skin laminate as there is minimal bleed through.


This versatile mat can also be incorporated into vacuum bagging processes as well as resin infusion.

CSM laid on a mould

CSM offers comparatively poor performance compared with higher performing and more expensive reinforcements (woven glass fabric, carbon fiber, aramid) but it is useful for increasing the thickness and therefore stiffness of parts in a cost effective manner. The strands leave a lot of space to absorb resin, which makes a stiffer, and heavier product.



On a side note - when working with fiberglass, you want to aim for resin to glass ratio with as low resin content as possible, but still ensuring a 100% saturated laminate. This ensures that the tensile strength of the fiberglass is complemented by the rigidity that the resin offers. Too much resin, leads to a brittle product which can easily break upon impact or fatigue over the lifetime of the product.


Woven Roving


Woven Roving is made from continuous glass fiber roving which are interlaced into heavy weight fabrics. You will find a variety of weaves that each serve a particular purpose. These weaves all behave differently, and it is important to look through your options before choosing one for your project. What one weave might be great at, a similar weave could perform very poorly at and vice versa. It’s important to know and understand these characteristics before you start laying down fabric. This will be addressed in another article entirely!


Woven Roving is compatible with all resin systems, and unlike CSM, with its chemical binder, can be used with epoxy. It is mostly used to increase the flexural strength of a product as well as the impact strength. The interweaving results in a strong fabric, but it does not mold to complex shapes as easily as CSM.


Woven Roving


Laying up a Kingcat hull with Woven Roving

stitched bi-axial

Stitched Fiberglass Fabrics are ideally suited for selective reinforcement. Fiberglass is bundled and stitched together, rather than woven, so there is no crimp in the yarn. As a result, these reinforcements offer optimized directional strength for their finished composite parts.


Stitched Bi Axial

These fabrics are ideal for wet lay-up and the infusion process, where fast wet-out and strength control are key—they're meant to be used with resins to create hard, composite parts and are compatible with polyester, vinyl ester, and epoxy. This fabric can also soak up a large amount of resin, which results in a heavy product. For this reason, infusion is ideal.



Bi-axial cloth is designed to add strength in two directions for finished composite parts. Thus, fabrics are anisotropic, or strong in only two directions. Fabrics need to be oriented so the fiber yarns run parallel to the expected loads. If extra strength is needed in a different direction, another ply must be added at an angle to the first. The most common angles are +/- 45 degrees.

The threads woven through the fabric makes this unsuitable as a skin layer, and it is advisable to use a layer of CSM first.



This article by no means covers every type of fiberglass application you can use. It would have to be at least one hundred times longer to cover every possible use and technique for a variety of situations!


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