It is a thixotropic thickening additive. Mix it into epoxy, polyester, vinyl ester, gelcoat, or paint to control sag, create a non-slump paste for vertical and overhead bonding, and stop resin from draining out of joints and fillets.

Add it slowly in small amounts. Start around 2 to 5 percent by volume and build to the consistency you want, from light sag control up to a thick peanut butter bonding paste. A little goes a long way, so add gradually to avoid over thickening.

Fumed silica works with epoxy, polyester, vinyl ester, gelcoat, and paint. It is compatible with two-part marine epoxies and with catalyzed polyester and vinyl ester systems, thickening any of them into a non-sag paste for bonding, gap-filling, and making thixotropic gelcoat and resin. Add it slowly, since a little sharply raises viscosity.

No. It creates a hard, dense bond line that is difficult to sand, so it is best for structural bonding and sag control rather than fairing. For an easy sanding fairing mix use a lightweight filler such as Q-Cells, 3M glass bubbles, or phenolic microballoons.

No. Fumed silica is non-hazardous and not regulated for transport, so it ships standard ground. It is a very light, fine powder, so the jars look large for the weight. Wear a dust mask when handling it, since the airborne dust is an irritant, but there are no hazmat shipping restrictions.

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.