Phenolic microballoons are lightweight, hollow spheres made from phenolic resin. They are commonly used as a filler material in boatbuilding and composite repair to create fairing compounds, lightweight putties, and to reduce resin density.

In boating, phenolic microballoons are often mixed with epoxy resin to create a fairing compound that is easy to sand and shape. This mixture is ideal for smoothing hulls, decks, and other surfaces before painting or finishing.

To use phenolic microballoons, mix them gradually into a two-part epoxy resin until you reach the desired consistency—typically a peanut butter-like texture for fairing applications. Always follow your epoxy manufacturer’s instructions for mixing ratios.

You can also use our Epoxy Calculator to help you determine the right quantities.

No, phenolic microballoons are designed for fairing and cosmetic work, not for structural bonding or load-bearing applications. For structural repairs, use a high-strength filler like colloidal silica.

• Extremely lightweight
• Easy to sand once cured
• Low density, which reduces overall weight of the project
• Excellent for fairing and cosmetic applications

They can be used with both epoxy and polyester resins, although they are more commonly paired with epoxy for superior bonding and moisture resistance.

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.