Why do we continue to paint cars with oil based compounds; plastics, solvents, resins, etc?
It's surely the least (or at least, not the most) efficient method for preserving ferrous metals.
Just +10.5% chromium content will resist corrosion under normal atmospheric conditions, and it'll look good. And if chromium is too expensive, it can be electroplated via electrolysis, as with gold-plated jewellery.
Supplemental quote regarding stainess steel, from Wikipedia:
"..stainless steels contain sufficient chromium to undergo passivation, spontaneously forming a microscopically thin inert surface film of chromium oxide by reaction with the oxygen in air and even the small amount of dissolved oxygen in water. This passive film prevents further corrosion by blocking oxygen diffusion to the steel surface and thus prevents corrosion from spreading into the bulk of the metal. This film is self-repairing if it is scratched or temporarily disturbed by an upset condition in the environment that exceeds the inherent corrosion resistance of that grade."
Otherwise, we could use any other malleable alloy that won't expand, or become porous, and flake as it oxidizes; or even something that seals itself off from the environment (like stainless steels), then induce the process. Again, if that's too expensive, an electrolytic process can be used, as with anodized aluminium.
These (and likely more) techniques, have additional practical benefits. Three immediately come to mind:
No runs or drips.
You can't paint what you can't reach. Want to paint the cavity between two sheets of metal (perhaps a bonnet/hood or trunk/boot-lid) so they don't rust again? Good luck with that.
Plastic (i.e. painted) barriers are likely around three orders of magnitude thicker than electrolytically deposited barriers. What? Consider the difference between a metre and a kilometre. That's three orders of magnitude, and the relative difference between microns & millimetres. And then? Paint a bolt. The whole thing. Yes, including the thread. Primer, several coats, and clear. Now do the nut. When they're ready, try uniting them without stripping the paint. You can't. In many cases (particularly with fine threads), the thread won't really resemble much of a thread at all; each layer brings the valleys closer to the peaks, and tolerance is exceeded.
Supplemental quote regarding electrochemical passivation & oxide coatings from Wikipedia:
"Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish. True gun bluing is an electrochemical conversion coatingresulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe3O4), the black oxide of iron. Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action. A distinction can be made between traditional bluing and some other more modern black oxide coatings, although bluing is a subset of black oxide coatings."