I used sandpaper to scrub them down first. That removed a lot of the surface rust. Then cleaned down the dirt/dust with white spirits.
Then 2 coats of hammerite smooth, hand-brushed on.
I wasn't particularly fussed with masking anything off, I just wiped off any excess with the white spirits. I thought it might look crap with paint brush marks but I actually surprised myself with how well they turned out!