It is fine cotton fiber that you mix into resin to make a tough, gap filling adhesive and fillet putty. It adds strength and abrasion resistance and is excellent for structural bonds and filleting.

Cotton flock makes a very tough, impact-resistant mix that shapes and machines better than glass-fiber fillers and is less itchy to work with. It feels heavier than microballoon fillers but is far stronger, which makes it a favorite for structural fillets and high-strength bonds in epoxy.

Cotton flock works with epoxy, polyester, and vinyl ester resin, and it is most common in epoxy filleting and bonding. Stir it into mixed resin to a smooth, tough paste. It is prized for strong structural fillets rather than fairing, so fair over it with a lightweight filler if you need a smooth finish.

No. Cotton flock is tough and not intended for fairing: it sands hard and ragged. Use it for strong bonds and fillets, then fair over it with a lightweight filler when you need a smooth, finished surface.

Yes. Cotton flock is non-hazardous and not regulated for transport, so it ships standard ground with no hazmat fees. It is a light, fluffy fiber, so a little fills a lot of volume; expect a bulky package for the weight.

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.