Modern cars are galvanized, which explains the huge reduction in automotive rust over the past 30 years. Modern cars still rust, but they rust a lot slower than older cars, which were not galvanized. 

They don’t rust because of the cathodic protection. Rust is chemically impossible. (There might be the smallest amount of inconsequential surface corrosion, in places, like near scratches, but it won’t grow. It can’t.)

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Essentially we’ve got the Japanese to thank for this. See, back in the ‘70s and ‘80s in Australia, the car market was essentially American. It was all Ford and Holden, which means: we did business the way America did business, and America was quite happy to throw car owners under the ‘rusty’ bus after just a few short years, rolling around.  

And then along came Japan - which has a lot of salty coastline relative to its land mass, and it started introducing galvanized cars in America. They didn’t rust as much, and consumer demand kind forced the hands of the US Big Three automakers to introduce galvanising. And the rest is history.  Fast-forward to today: all new cars are galvanized in developed markets, globally. Even Australia. 

So, stone chips and minor scratches are not the problem they once were. You can repair them for aesthetic reasons if you want to, but there’s no need to obsess about it from a rust protection point of view, because of the magic frog-jolting action of cathodic protection.  Major damage - where lots of zinc comes off, in the manner of frogskin in the Galvani household on a Friday night, after a few Chiantis, that’s gunna rust. 

So, should you neck a few scoobs one evening and decide to buff your wanking chariot with a 40-grit flap-disc, until the battery in your angle grinder sucks on a dry tank, that’s gunna be a problem.  The best corrective measure there (aside from jumping in a time machine and telling your parents not to breed) is to prime it all up with a zinc-based primer, and re-paint, preferably with a pressure pack can (or cans).