It is finely milled glass fiber used to thicken resin and add reinforcement. It improves the strength of fillers and adhesives and helps prevent cracking in thin bond lines and fillets.

Milled fiber is a fine, short glass-fiber powder that blends smoothly and is easy to work, while chopped strand uses longer fibers for greater strength. Milled fiber sits between a thickener and a structural filler: use it to add body and modest reinforcement, and step up to chopped strand where you need maximum strength.

Milled fiber mixes into epoxy, polyester, and vinyl ester resin. It blends smoothly into any of them to add body and reinforcement for bonding and thin laminates. Stir it into mixed resin to the consistency you need, from a thickened resin up to a stiff paste, just before you lay it up.

It sands moderately: firmer than a fairing filler but workable. For a fully fair finish, blend it with or top it with a lightweight fairing filler. On its own it is better for adding strength and body than for a smooth, finished surface.

Stir milled fiber into mixed resin until you reach the body and reinforcement you need, anywhere from a lightly thickened resin up to a stiff paste. More fiber adds strength and stand-up, less keeps it flowable. Add it gradually and mix to no dry clumps before you use it.

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.