Glass Bubbles are hollow glass spheres that act as a lightweight filler. Chemically composed of glass, they are harder than microballoons, making the resulting resin mixes noticeably more difficult to sand. However, this hardness also contributes to a more waterproof filler mix, often making them the preferred choice for below-waterline applications on boats.

Significantly less expensive than microballoons, glass bubbles are ideal when easy sanding isn't the top priority. 

Hollow Spheres (General)

Both Glass Bubbles and Microballoons are hollow spheres designed to increase the volume and reduce the density of any resin system. They are commonly used to create lightweight adhesive mixes, as well as filling and fairing compounds.

• Glass Bubbles: White microsphere filler powder used to make glues or paste fillers.