Red oxide by Witham Paints is very good, I painted my old chassis with it and then hammerite on top 5 years ago. The hammerite chipped off in less than 2 years, the red oxide is still there despite being soaked in clutch fluid!
Failing that the tractor spares company Sparex supply enamel paint in any tractor colour you can think of, it does on quickly (spray or brush) and takes a good belting with a hammer to chip it.
Harry