fluid film
it must be those tires, most 240s i see with those end up rusting out.
the following MAY be an opinion.
I was trained and worked doing auto body and panel fab, and i have done this "both" ways.
this spot sucks, lots of seam sealer and undercoating fun.
the proper way is a bitch, takes time, and you really have to love the car, hate yourself, or both. it will take toxic chemicals, the right tools, and an understanding of the process as well. experience does not hurt either.
GOOD lasting body work takes time and
WILL be expensive, and if it is not done well, or especially, not prepped well, especially with a car you are salting, the rust will just come back. if you take it to someone and it seems to cheap to be true, it probably will be...
anyway... there is an alternative you just may not like it.
while i can and have done this on a professional restoration level, i just don't really care to have perfect cars myself. i have other places time and money need to go.
if you can live with your car being/looking "rusty", but want to slow the rust to a near stop, this is what i do these days on my own stuff that is non structural.
remove scale and anything loose. wash with soapy water and scrub, rinse with warm water,
completely dry it with heat gun, apply fluid film, then hit it with heat gun again, repeat application and heating until saturated, then come back later and do it again if it needs it.
done!
now enjoy the smell of sheep, barnyard, apricots, and bandaids (almost like a good scotch) that is fluid film.
or if there is a hole that needs covered at that point i make a crude patch out of whatever metal i have around, cut extraneous body parts that may be in the way, fit it, drill it for rivets, apply metal roof sealant on the outer inside edge, stick on and rivet, make sure to wipe and clean anywhere you have the fluid film dripping before sticking the panel over.
you could also do the same and use POR15 instead, i have a small flock of sheep so i prefer the fluid film.
here in NH, state inspection states we cannot have "holes" in our cars, but they still keep throwing down that darned salt.
i can throw up pictures later of the last 240 i "properly" fixed, and the last one i "slopped"
the wife took the sloppy one to work though.