I have a MING reconditioning manual from the 1970's and they said to spray clear lacquer on the underhood components after thoroughly cleaning and painting them. I did that myself on a car I flipped back then and got nice results.

Tempo paints (they manufactured a line of color-matching spray cans like Dupli-Color) even had 'reconditioning paint' that was a very low-gloss black paint product for underhood use.

I seem to remember a product in a metal one-gallon container called Do-All or something reasonably close to that.