The house we inherited had about 1/2 a mile of pipe that need to be repainted. It was white but we turned it to black. Here's what I did and it worked well. Maybe this will help someone.
First, did alot of welding repairs, used ospho on any heavy rust areas. I then used drywall sanding screen to knock off loose paint or heavy dirt. Ater that I power washed the whole thing. Used Sherman William's metal paint and applied with a car washing mit