Phenolic microballoons make a low-cost, ultra-light fairing and void-filling putty in epoxy or polyester. Sand very easily; cure reddish-brown, so best under paint. Phenolic microballoons are the classic reddish-brown fairing filler: they bulk out epoxy or polyester into a light, sandable paste for shaping and smoothing on hull and deck. They sand easier than fumed silica and add no structural strength. Compare your options in microballoon vs Cabosil fillers. Specs Material hollow phenolic-resin microspheres, grade BJO-0930 Color reddish-brown Particle size mean approx 60 microns True density approx 0.23 g/cc Crush strength 350 psi average Resin epoxy, polyester, vinyl ester Sizes 1 quart, 1 gallon, 4 gallon, 26 lb box Origin USA; non-hazardous, not regulated for transport Phenolic is the economy microsphere and the tough one: rated 350 psi average crush strength, so the mix stands up to hard troweling and power sanding. The trade is color and weight. It cures reddish-brown, so use it under primer and paint; 3M glass bubbles are white and lighter (0.125 g/cc true density) for fairing under light gelcoat or clear finishes, and Q-cells are the off-white middle option. All three plus fumed silica are in fillers and additives.
