I thought it was the other way around. Older plastics are less adulterated plastics and could use some help, where the new stuff is poly blend stuff that works well with poly urethane paints. I mean, think of a current bumper cover being pliable, and an old piece of plastic trim that shatters when it breaks.
There are some nylons that old formulas of epoxies will actually dissolve. Customer uses nylon caps to cover screws and wanted us to paint them with a CARC coating. I had to prove to them that it is not made for plastics as the epoxy and polyurethane topcoat just flaked off after pitting the part. Epoxy hardens due to a chemical reaction, and what happens to the plastic during that reaction is what you need to learn.
Try something inconspicuous first or use some cheap epoxy that might not hurt the plastic. Honestly, there are some rattle can paints that are made just for plastic from Rustoleum or Krylon.