I painted my brake drums virtually from new with a couple of coats of Hammerite and after 3.5 years and nearly 20,000 miles of year round use they are still fine as the drums don't get that hot. Ditto my Land Rover one's which were treated many years ago. If there was surface rust would treat first with Kurust.
Just treated the diff casing similarly which comes with the thinnest of paint coatings from new.
