Chopped strand mat (CSM) is a non-woven fiberglass reinforcement made from randomly distributed glass fibers held together with a binder. It’s commonly used in composite layups for its affordability, ease of use, and excellent bulk-building properties.

Most CSM is designed for polyester or vinyl ester resin because the binder in standard mat doesn’t dissolve in epoxy. However, epoxy-compatible chopped strand mat is available and should be used if you’re working with epoxy systems.

On average, you’ll need about 1.2 to 1.5 times the weight of the mat in resin. For example, 1.5 oz CSM will require roughly 20–25 oz of resin per yard. Use our Epoxy Calculator to estimate your resin needs, adjusting for polyester if necessary.

Absolutely. It's commonly used between layers of fiberglass cloth or woven roving to add bulk and prevent print-through in gelcoats or top layers.

Yes, it's important to use a fiberglass roller to eliminate air bubbles and ensure proper wet-out during lay-up. This improves the strength and finish of the final part.

CSM can be easily cut with scissors or a utility knife. For complex curves or better blending of layers, it can be torn by hand to help it conform to the mold shape more easily.

CSM is ideal for hand lay-up and open-mold applications. It’s used in boat building, mold making, and general fiberglass repair to create strong laminates, fill voids, and provide a smooth finish when layered with polyester resin.