Bondo® glass reinforced filler is great for use on fiberglass, wood, metal, aluminum, SMC and masonry and is compatible with all paint systems. 1/2 in, interlocking fiberglass strands create strength and make Bondo® glass the ideal filler for repairing small (dime size or smaller) holes or rust-outs. The fiberglass strands in the filler allow you to cover holes and eliminate the need for backing strips, fiberglass mat or cloth.