Por15 is your best bet. Used it to do an entire (as much as I could physically reach) 3rd Gen 4runner frame.

Definitely suggest grinding off the old finish and any rust. Cleaning and prep is your best friend. If you don’t entirely strip it, I would recommend clearing the rusted area the best you can, getting a rust converting spray to cover it, and then do por15 over that if you go the por15 route.