Originally rumoured to contain tiny iron spheres. The theory was known to the Germans in ww2.MM, I bow (not facetiously) to your hands-on experience.
I'm well aware of the anti-RADAR countermeasures incorperated in the design. I'm suggesting that for a RADAR c/s, the '71 was all but invisible, but across the spectrum, it appears that most knew roughly WHERE they were but couldn't pin-point exactly. Hence, alomst 1,000 SAM's being fired at them, and not a single hit, never mind actual damage caused. Apparently, the heat signature identified where the airframe was, more than RADAR.
The closest I have come to a '71 was at Duxford. The skin feels... rubbery... but slippery at the same time. Whether this is the actual RADAR-absorbant paint they used or not, I cannot tell.