Removed fuel tank have some rust what would you do

[Missyblue]

As I mentioned we were excited to get power in cab fuel tank removed but underneath I definitely have some light to moderate rusting issues with some pitting. The truck is already been painted so not sure sandblasting is an option. I've read about different rust treatment things such as POR 15 and Eastman products just curious if any of you have opinions or thoughts? I've also read about some gels and products tht are supposed to help dissolve the rust. Or do I attack it with some sort of sanding/grinding blade. Oh and light rust under the old headliner too