Bondo Short Strand Fiberglass Reinforced Filler makes an excellent foundation for hole and rust-out repairs without the need for backing strips, fiberglass cloth, or mat. It contains short, interlocking fiberglass strands and fibers that make it stronger than regular body filler. It is also slightly thicker and provides a fast drying time, making the filler less messy on vertical surfaces like car doors. Fill stage of 3M Body Repair System.