3M SJ3550 Dual-Lock Fastener. 25mm x 45.7m Black. 3M Dual Lock SJ3550 is UV and plasticiser resistant. Higher bond strength and high performance for external applications. Self-engaging (Sticks to itself). As authorised distributor for 3M, Directa can help you with any of the Dual lock range, please click here to see the full range of 3M tapes.