They are engineered hollow glass microspheres used as a lightweight fairing and bulking filler. Mixed into resin they produce a low density, easy sanding compound for fairing and shaping.

We supply 3M grade K1. The bubbles have a median particle size of about 65 microns, with a typical range of 30 to 115 microns and an effective top size near 120 microns. True density is 0.125 g/cc and the isostatic crush strength is about 250 psi, which makes K1 the lightest and easiest sanding grade in the 3M line.

G18/500 is a denser, stronger grade at 0.18 g/cc and 500 psi crush strength, built to survive high-shear mixing and higher-pressure work. Our K1 is lighter and softer at 0.125 g/cc and 250 psi, so it sands more easily and suits general fairing. Particle size is similar between the two, so the real difference is density and crush strength, not size.

They are precision made soda lime borosilicate spheres with tightly controlled size and crush strength, so they give consistent density and durability in the mix. They are white, which keeps the cured filler light in color.

Yes. 3M K1 glass bubbles sand and feather easily, which is what makes them well suited to fairing large surfaces with minimal weight gain. The soft K1 grade at 0.125 g/cc cuts faster than denser grades, so a fairing mix made with them blocks down quickly without loading up your paper.

3M glass bubbles mix into epoxy, polyester, and vinyl ester resin. They are most often used in epoxy fairing compounds, where their low density keeps weight down while building a sandable, void-filling body. Add them to mixed resin until you reach a light, creamy, easy-to-spread fairing consistency.

Yes. 3M glass bubbles are non-hazardous and not regulated for transport, so they ship standard ground with no hazmat fees. The bags and jars are bulky for their weight because the product is so light, but there are no 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.