Remember POR-15 stands for paint over rust - remove the scale and wash the loose dust off and paint away.
Look at the Material Safety Data Sheet for Marine Clean - it is related to oven or grill cleaner. Marine Clean is meant to cut oil film and slightly etch (and rust) bare metal to provide something for POR-15 to better key into, it is not a rust remover so do all the mechanical brushing etc. before the Marine Clean process.
I had a bunch of borderline bad metal and went a little gonzo around any of the damaged areas... I found auto body shop tools worked the best - First 3" medium wire brush then 2" pancake discs of coarse and medium Scotch-Brite both chucked into a high speed die grinder to give you an idea how to automate the process.
My '73 27' frame paint was a tar based paint that covered me in a creosote smelling brown stain - especially with the air tool discharging some condensate. The wire wheel stripped it off the best - the pancake pads worked best grinding down surface rust. Beware wire wheels - they throw bristles (2-10mm needles) and if you are crawling around working on your back etc. you will get bit. I made a magnet sweep after I wore a bristle under my skin for three days.
The places inside the main frame rails without damage got wiped down with a mineral spirits cloth and painted with flat-black Rustoleum spray paint - not everything needs POR-15 armor - and the POR-15 may not bond to the non-deteriorated paint, I see one spot there was some peeling (disclaimer: noticed after I welded nearby it).
Anyways - MC cleans oil and grease and etches iron. Careful rinsing it off if your old floor wood is exposed, the wee beasties in the old wood may be craving a drink of water...
The days are short and the night is long and the stars go tumbling by.. . ~Airstream~