Removing 51 year old rust from my project moped 

As a antique firearms conservator i can tell you that there is a trick to convert rust (brown oxide) to black oxide (gun blueing) by steaming or boiling in demi water. The Chromium underneath the rust will become to the surface while the brown oxide dissapears. Black oxide is a protective layer. :))

Wet tee shirt and sand works great too, been doing it this way for years,