Microsphere powder—often called glass bubbles or micro-balloons—is an ultra-lightweight filler added to epoxy or polyester resins to reduce weight, lower costs, and create a highly sandable paste. Why Use Microspheres in Resin?Weight Reduction: Because they are hollow, they significantly displace resin weight, creating lightweight parts Easier Sanding: Adding microspheres turns hard, cured resin into a soft, easily sanded material, making them ideal for gap-filling, fairing, and repairs Application: The paste can be used for gap filling, lightweight casting, or sculpting. Note that microspheres will ruin optical clarity, resulting in an opaque, milky-white, or colored finish.