Hello Antony
You can see my efforts at rusting ……….on my Great Eastern and Jeanie Deans at war albums………………I use artists acrylic paint…Burnt sienna…..yellow ochre and raw umber
You will also need to add plenty of dirt……………in all corners and cracks and around all deck fittings
Always paint the rust in vertical streaks from the anchor ports etc and along plate joints etc and use a variety of colours
Post us a picture when you`ve done it?
Bob