Just remembered Frost. They sell a range of products. Eastwood are one who make good paints but not cheap! I used some on my first Mog chassis. They sell Por and their own brands as well. I currently have a tin of Frost chassis paint used over encapsulator and primer. I have used it on the rear axle and it is lasting really well. The solids do settle out so a good stir is needed if not used for a while.
https://www.frost.co.uk/Years ago when in a steel works lab we made our own using phosphoric acid to create a phosphate coating. The recipe is only vaguely remembered now.
