Check the link from the 1959 Schwinn Reporter article and take the time to read directly from Schwinn
https://thecabe.com/forum/threads/schwinn-paint-process.86284/ where they state in black and white, "coated with aluminum enamel" "and baked for 19 minutes". Wouldn't that be more obvious than all of this conjecture? Plus I have never seen any paint listing or can, samples etc. of gold underbase in any catalog or collection anywhere.
Paints have chemicals as ingredients that react to environmental circumstances such as time, temperature, UV exposure, etc