Fuzzy Couplers started as a mistake. An order of several hundred 4" x 7" couplers was incorrectly made out of spec - they were too tight to fit into their intended airframes.
After peeling them to make them fit, we found that epoxy really adheres well to the fuzzy surface. We started to use them as stiffeners in our models - essentially doubling the thickness of the airframe side wall - greatly increasing airframe strength. Their rough surface creates an excellent bond to epoxy. It's become a standard design element in SBR kits to include airframe stiffening coupler tubes.
Now you can buy them individually - at a great price! Fits SBR 4" Airframe Tube. 4" x 7" Coupler