How do I get rid of this rust in the trunk?

Ospho doesn't need to be neutralized. 24 hours after application scuff off any fuzz that developed and you can prime and paint over it. Only downside I've found is it isn't as effective on vertical surfaces because it is thin and drains off and doesn't have enough working time. Sometime two applications are needed for heavier rust if you don't knock all the scale off.

That rust was likely there before the POR was applied. It's job is to stop the reaction and give a stable base to paint over without all the mechanical work required to remove the existing rust. If there isn't any rust, POR doesn't adhere very well.