At a rough guess, the original enamels may well have gone straight on to bare metal? Would love to know more. I use acrilic enamel with a light coat of high build primer/surfacer in black under it, which I think gives a deeper red and shows up less when chipped or scratched.

I haven't seen any signs of primer on the black or red parts that I have. However I have seen light orange as a base for the red parts on 2 different -28 101's that other persons owned. Impossible to tell if they where factory painted or shop painted but both had same colour for what I could see and was old and professional job anyhow.