Chopped carbon fiber is added to epoxy, polyester, or vinyl ester resin to boost strength and stiffness without the bulk of woven fabric. Shops use it for reinforcing fillets and bond lines, casting small structural parts, and molded forged-carbon parts where you want the carbon look and high strength in a flowable mix. It is reinforcement, not a fairing filler.

Yes. Chopped carbon fiber works with epoxy, polyester, and vinyl ester resin. Epoxy is the usual choice for structural carbon work because it bonds better and is less brittle. Stir the fiber into mixed resin to the consistency you need, then wet it out fully so there are no dry fibers left in the layup.

Shorter 1/4 inch fiber blends smoother and works into tight fillets and thin sections more easily, giving a cleaner finish. Longer 1/2 inch fiber carries more load and adds more crack resistance, so reach for it where structural reinforcement matters more than surface smoothness. Many shops keep both and pick by the job.

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.