3M 1157R electrical tape is a 100% rayon fiber backing coated on one side with an acrylic pressure sensitive adhesive system for an overall caliper of 4 Mil. The porous rayon mat and adhesive are both permeable to gases and liquids. - 1157R was specifically designed to allow thorough penetration of the impregnating varnishes inside bobbin-wound coils. Porous construction allows varnishes to penetrate thoroughly.