What's the Best Way to Prep Unknown Condition Steel?

Hi,
I pan on starting body work soon on my 71 Scamp and I have questions about prepping the metal. I've never done this before so it is a learning experience. My C pillars are solid but rusty due to being an old vinyl top. I will do my best to remove rust from this area and the roof. I'm worried that rust may still lurk in pitted areas. What's the best form of metal prep that will work on the steel and convert or remove rust?

I know there are different options, just looking for people's opinion. Does phosphoric acid work well?

Can I epoxy prime over POR15 metal prep if I use that?