I think the hardest type of paint you can get in spray cans is epoxy type.
Dries much longer, but has more solid build and rock hard.
However, the harder and thicker the paint, the more likely it will chip !
Hammerite, epoxy and other hard finishes will eventually crack and ship right off to the primer on metal outdoors.
I had good luck with rustoleum exterior paints, which come in all colors and need little prep work.
They actually recommend spraying right over the rust and seal it with the paint.
Like all paints, it will eventuall come off, when the paints adhesion strength starts to diminish from weather and baking in the sun.
As with all paint jobs - the better your prep work, the better and longer lasting the finish.
