RPF PREMIUM MEMBER
What about using Micro Marks TCR-40 Tire casting rubber? I have parts that I molded over a decade ago and they show no signs of deterioration. The dark marks are "glyphs" that I was using heated instruments to make impressions. That being said, the rubber can be dyed black and I just checked the originals (now 11 years old) and there is no sign of any deterioration (no brittleness or gumminess).