I have found that aluminum rust paint that you apply by brush is about as good as it gets and lasts a long time too. Works well at stopping and preventing future rusting.
I have found the same thing. Much better than "cold galvanising". I tried cold galvanising, and within a few months spots of rust were coming through. Plus, it is an ugly flat grey, that doesn't match the galvanised surface at all. That stuff is a waste of money and effort.
OTOH, aluminium paint blends in with the original galvanising very well. After it weathers for a few months, it is hard to tell the difference. My tower is mounted on an old base insulator originally used with an AM broadcast tower. The steel end bell castings were not galvanised at all. When I installed it 30 years ago, I painted all the bare steel with aluminium paint, and some of the galvanising on the tower began to show signs of rust before the painted castings did. I think I may have re-painted the castings once since I put up the tower. I also used it to touch up some ding marks where the galvanising was chipped off, as well as some outdoor steel hardware I cut with a hacksaw, exposing the ungalvanised interior. Now, you can hardly tell where the touch-ups are.
I would say that aluminium paint is about the closest thing you can come to galvanising steel without actually having it hot-dipped. Best of all, if there is light surface rust, the paint soaks down into the pores of the rust before it hardens, and once the paint is thoroughly dry, the rust acts like a primer, making for very good adhesion, almost impossible to scrape off.