I'm taking advantage of the weather to paint my garage doors - long
overdue.

The doors are hung on hinges like these:
http://products.ironcdn.co/570883.jpg
Unfortunately they never got painted when originally fitted and are now
very rusty (surface rust only; they are perfectly intact and strong.

I know that the best way to sort this out would be to remove the doors
altogether and unscrew the hinges, however that's really not practical for
several reasons and isn't happening, So, given that the hinges are
definitely staying on and want painting (black), what's the best way
forward? Hammerite, which says to paint straight on to rusty steel without
brushing first? Or treat the steel with some chemical, and then prime and
paint as usual?

Thanks

--
David