This guy has a good video explaining the steps, I truly believe ospho is great, I use it on every thing that has rust, I wire wheel the area , ospho it, then scotch bright it, then ospho it again, after having watched the video I now try to re activate it and clean it off, , here is the link, the second and third video shows the results,
https://www.youtube.com/watch?v=pgnR7gAeOQ0