Phosphoric acid metal treatment. Goes under various names. Marine application is called Osphocote, theres a brand you can get at body shops,but the name escapes me.
You can use it directly on surface rust, but check paint compatability in an inconspicuous spot first. Pick at the loose rust with a sharp object first, then apply.
Its water thin, so you can use it to flush the insides of all the tubes to rust-proof the bike. Be sure to drill a big hole (app 1/4 inch) in the bottom bracket to act as a drain for the frame.
Il faut de l'audace, encore de l'audace, toujours de l'audace
1980 3Rensho-- 1975 Raleigh Sprite 3spd
1990s Raleigh M20 MTB--2007 Windsor Hour (track)
1988 Ducati 750 F1