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.

Each West System filler is tuned to a different job:

• 406 Colloidal Silica: the versatile thickener for bonding and filleting; controls sag on vertical and overhead work.
• 404 High Density: the strongest, for hardware bonding under high load.
• 403 Microfibers: add toughness for structural filleting and gap-filling.
• 405 Filleting Blend: a natural wood-tone fillet that sands easily.
• 410 Microlight: the low-density fairing filler that sands the easiest over large areas.