I'm a bit lost to the final answer/solution.
Here in OZ, we don't get the salt issue, re-guarding snow etc.
Best thing to do is stop rust.
A few ways to go about it..
One under valued way is to buy a zinc anode meant for marine applications!!
Usually fitted to outboard motors!
Electrolysis!!
Google it!
You will be surprised at w h at at an anode can do!
Talp n to your chassis!
Cheap as!
Have you ever seen an old motor covered in with oil leaks... rusty ?
NO!
A penetrating oil here in OZ, "inox", like a light sewing machine oil.
I'm use once a year, in an engine cleaning gun, I spay the underbody with it.
Then feather it out with an air pistol.
Don't forget get the chassis rails! Drench them!
Squirt some in from both ends, and then use compressed air to feather it out thoroughly .
A good penetrating oil will loosen up grime and crap.
Far better option in my experience.
Tar, synthetic coatings won't do that.
But honestly for $10!!!
Google out board zinc anodes re- electrolysis..
And how they work!!!!