[Synchro909]

Back when I was a kid in my teens, I needed a bicycle to get to school. My father gave me his old one and I set to getting it going again. The bracket he had made for the headlight was rusty and I wanted everything to be "just so". Dad gave me a bottle of phosphoric acid and told me to soak the bracket in it till the morning. It came out as clean as a whistle and when it dried, there was a white powdery residue on it. Dad told me to leave it like that and I'd have no problem. It was and there was no sign of rust on it for about 15 years.
I've never heard of Ospho (probably not available here) but if it is diluted version of what I used, it should be OKish but a more concentrated version might be better.