This is quite a tricky one to fix, because paint doesn't look as good as a nicely machined hub, and doesn't last well in such an exposed position. Any work to fix can be time consuming and fiddly. Normally on hubs I would sand off the rust using fairly fine wetndry. 600 grit will do this if it's not too bad. You might also try vinegar as a lubricant, or alloy wheel cleaner because a light acid etch will help to shift the rust.
Small rusty items can be left to soak in vinegar over several days and the result is usually impressive, but not wheel hubs!.
If paint is to be the fix, then etch primer first with a coat of aluminium silver, then lacquer. Mask the suspension & caliper off. The amount of time that this will last is wholly dependent on how well you prep it. Other paints will also do the job, but beware of the hammerite types because they go on thick and can crack easily.
