I used Nason synthetic enamel for years with Nason Qwik-cure hardener. Works great. I think you can still get the black. It's about $90 a gallon. Finishmaster sells it. Polishes out like the best base coat clear coat.
If you can't get the synthetic enamel, acrylic urethane is pretty similar, and not too much more expensive. I quit PPG years ago because it's just plain overpriced. By the way, I painted my GN 15 years ago with nason synthetic enamel, and took that photo yesterday. 😉
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