3M SCOTCHLITE GLASS BUBBLES - Glass Bubbles are hollow unicellular, microscopic spheres composed of water, insoluble chemically stable soda-lime borosilicate glass. This filler/resin extender can be used in mixing of marine putties and fairing compounds, as well as in fiberglass reinforced plastic layup. These bubbles offer light-weight, minimal shrinkage and easy sanding. Product is sold by the 50 lb. box.