Rust Repair

Hello All,

Like most of you when you started I'm comfortable with mechanical and less so with body work. My recently acquired 70 Swinger is in great shape but a few spots of scaling rust has happened on the body. I've been given two different pieces of advice.
1st - To sand down to bare shinny metal and primer. Some of the scale is pretty deep and requires a lot of sanding which can create too much heat. Most of this rust is on the c pillar where the roof meets the rear quarter panel. My Swinger has filler in this spot.
2nd - To sand just a bit to score it and use a POR-15 type rust preventer. This method keeps more metal and more structural integrity.

What are your thoughts?