Alkyld enamels, such as Rustoluem, two biggest faults are chalking due to UV light exposure (from the sun, etc.) and chemical resistance. Adding some hardener will increase the chemical resistance considerably. Parts underneath the car receive minimal UV light exposure, so Rustoleum with some hardener added should hold up well. This type of finish would be acceptable for a summer time week-end car.

Obviously, a person could spend more money and use an epoxy primer and urethane paint or power coat chassis parts. These are superior coatings. I would seriously consider these finishes for vehicles driven on salt treated roads.

On a vehicle which is driven, any chassis paint or coating will be chipped and scratched over time. So a finish which can be easily repaired is usually a good idea for a frequently driven vehicle.