M-bodies can and will rust out – but, overall, are way better than F-bodies.
I "think" the reason is the F-body was the low end car when it came out in ’76. That position went up a notch when the L-Body came out (Omni/Horizon) in ’78 – but quality of F-bodies didn’t change, any.
My opinion as to why, is they didn’t take time to dip cars correctly (if at all), didn’t take time to prep car before paint and/or didn’t apply as much paint as they should have when cars were built – like they did with M-Bodies (or other bodies).
Pretty much all ’76- (to ’79?) F-body fenders will rust out on top side. The recall fenders are to fix that concern. Even my ’77 Volare wagon, with 40k miles and has been kept inside of a garage 95% of its life, has rust through holes on fender tops.
Trunk rust can be an issue – but not as much as A-body or earlier B-bodies are.
Seems like front frame rails and floor pans might be (I think) the big rust areas.
Floor pans are not being repop’ed (that I know of) but can be found.
Saf-T-Cap makes front frame rail caps for our cars – which I’ve heard good things about (but not personally used).
Floor pan rust can normally be seen under the car and is normally at any of the 4 lower outer corners (of floor pan). A camera picture from the 4 corners will usually tell you if floor pan has rust damage. The front frame rails take a little bit more effort to look at – but can still be done without jacking car up any.
M-bodies, for the most part, do not have a rust problem – except for extreme conditions.
F-bodies, well – are not in the same category – even though the chassis/metal body are exactly the same (except for outer skins and such).
In '80, the F-body fenders are the same as M-body ones are.
BudW