I really wish I could help more but rust is something I have always ran away from. My 69 Camaro gave me some experience (free classics are hard to run from) but only around the rear window - not as important as the floorboard (well unless its a Nissan Altima where floor boards falling out are considered normal even in Florida).
I was made to etch (applied acid), removed loose metal and old coverings (wire brush) and neutralize (apply alkali) before my body guy would even let it into his shop to start his prep.
that was thirty years ago and from what I've read many rust coatings today can just wire brush and spray. Just pay attention to seams on floorboards that where they start forming the trap door that would have ya dragging your toes... ANY ability to poke through with a scribe should be patched.
J Wm Bishop EA, ASADE
Classy Chassis Customs. Because the only thing that matters is what's UNDER the dress ...