I've used two coats of Hammerite Smooth on steel car wheels, without
any undercoat, as instructed on the tin. Okay for a year or so, then
the rust comes through. More recently I've used Hammerite Kurust (from
Halfords), then Red Oxide, then a single coat of Hammerite Smooth
(ensuring that any missed areas are touched up within 8 hours) on iron
railings with excellent results. All the old paint must be chipped off
with a hammer beforehand to bare metal. Possibly two coats of
Hammerite over the Kurust would be as good but Red Oxide is a darn
site cheaper! Alternatively ordinary gloss could be used over the red
oxide although it may need two coats. Hammerite's "Direct to Rust
Metal Paint - 5 Year Protection" claim is fantasy - I'm surprised that
they don't advise using their own Kurust product first.