How do I paint the steel crossbeam on a colorbond fence?
As suggested, use sand paper or steel brush to clear any rusty bit
Apply some(not too much) rust  remover / preventer - let it dry naturally (30minutes - 2hours?)
Paint with steel primer (usually grey) 
Following the drying time guidelines of the primer apply your top coat of enamel or whatever you want.


Do not flood the steel with rust preventer. Let it dry and wash your hands properly. If the rust remover makes white bulges just remove it with a another tool. Do not touch with your hands to avoid a) acid burn, b) oil residue on steel

We use to do that in South Africa on all gate installations because summers were extremly dry and some winters could be torrential with flash flooding. This method lasts years and years.
(It is a method borrowed from the automotive industry - in its simplistic form)

Good luck