Since nylon doesn't adhere well to almost anything, you may not even need to coat it. However, I have used talcum powder as a mold release for RTV mold making. Also, Krylon primer does bite a bit into nylon, and I have painted nylon parts using it. Paint is not the most tenacious, but should last long enough to create the mold. I would follow the primer with a gloss paint before setting up the mold.