Bondo® Glass Reinforced Filler is great for use on fibreglass, wood, metal, aluminum, SMC, and masonry and is compatible with all paint systems. Half-inch, interlocking fibreglass strands create strength and make Bondo® Glass the ideal filler for repairing small (dime size or smaller) holes or rust-outs. The fibreglass strands in the filler allow you to cover holes and eliminate the need for backing strips, fibreglass mat, or cloth.