Firstly that is one unbeautiful box folder.
Forget red oxide primer it is old technology. Bondaprimer is the best I have found, it looks like red oxide but has anti rust bonding and etching agents and can take any kind of paint. I could go on but the proof is that I have given one coat on steel outside and never got around to applying a top coat, 5 years later and no rust. (usual disclaimer). I to have long given up on Hammerite it is too much of a pain for me to use.