1/2" chopped strand fiberglass is made from short lengths of glass fiber, each cut to approximately half an inch. It is used as a reinforcement filler in resin systems to improve mechanical strength, stiffness, and bonding in structural and repair applications.

1/2 inch chopped strand fiberglass reinforces epoxy, polyester, or vinyl ester resin for bonding, gap-filling, transom repair, stringer work, and any layup that needs added strength and durability. Stir it into mixed resin to make a tough, fiber-rich paste that grips into corners and bond lines where cloth will not lay down.

It works with epoxy, polyester, and vinyl ester resin, which makes it a versatile reinforcing filler for most marine composite work. Epoxy is the pick for structural bonds and wood, while polyester and vinyl ester suit production and gelcoat repair. Mix the fiber into catalyzed resin just before you need it, since pot life drops as you add filler.

Add the chopped strand gradually into fully mixed resin while stirring to distribute the fibers evenly. Continue adding until the mixture reaches your desired thickness—typically a stiff, trowelable consistency for bonding and structural filleting.

For structural filler applications, a good starting point is 1 to 1.5 quarts of chopped strand per 1 quart of resin. Adjust based on the desired consistency and level of reinforcement needed.

Match the filler to the job, and keep strength fillers below the surface with fairing fillers on top:

• Bonding and structural fillets: use a high-density or fiber filler like colloidal silica (Cabosil), milled fiber, or microfibers, which cure hard and strong.
• Fairing and shaping: use a low-density filler like phenolic microballoons, glass bubbles, or a fairing compound, which sand and feather easily.

Adhesive, high-density fillers like colloidal silica and microfibers make a hard, strong mix for bonding and structural filleting, but they are tough to sand. Fairing, low-density fillers like microballoons and glass bubbles make a light, soft mix that sands and feathers easily for a smooth surface, but they are not structural. Use adhesive fillers for strength and fairing fillers for finish.

There is no fixed ratio. Mix the resin and hardener fully first, then stir in filler a little at a time until you reach the consistency the job needs:

• Ketchup consistency: for a coating or pre-coat that still flows.
• Mayonnaise consistency: for bonding parts together.
• Peanut-butter consistency that holds a peak: for filleting and gap-filling.

Low-density microsphere fillers sand the easiest. Phenolic microballoons, 3M glass bubbles, and lightweight blends like our fairing compound or West System 410 Microlight feather to a fine edge and carve with little effort once cured. They trade strength for sandability, so use them above the structural layer for shaping and smoothing, not for bonding or load-bearing fillets.

Some are. High-density and fiber fillers like colloidal silica, milled glass fiber, and microfibers add real strength and are made for bonding and load-bearing fillets. Low-density fairing fillers like microballoons and glass bubbles are not structural; they are for shaping and fairing only. Build strength with a high-density filler first, then fair over it with a low-density filler.