M-D Building Products Gray Polyethylene Caulk Backer Rod For Driveway and Sidewalk 20 ft. L x 1/2 in. M-D Building Products 71480 Backer Rod for Gaps & Joints M-D Building Products 71480 Backer Rod for Gaps & Joints Features: Non-absorbent, closed-cell polyethylene pre-caulking material. Use before caulking to fill gaps, Prevents 3-point bonding.1/2" x 20', Caulk Backer Rod, For Gaps & Joints, Non-Absorbant, Closed Cell Polyethylene, Pre-Caulking Material, Use Before Caulking To Fill Gaps & Openings Larger Than 1/2" x 1/2", Prevents 3 Point Bonding. Poly Foam Caulk Saver acts as a filler in large gaps before applying caulk. Insulates and cushions. Use in pavement cracks wall joints and foundations. Use before caulking to fill gaps and openings larger than 1/2" x 1/2" Prevents 3-point bonding. M-D Building Products 71480 Backer Rod for Gaps & JointsM-D Building Products 71480 Backer Rod for Gaps & Joints Features: Non-absorbent, closed-cell polyethylene pre-caulking material.