Yup. Condensation is the big enemy. Car covers, unheated garages are little help against that source of moisture and it gets in everywhere!
Carpet with the jute? backing is pretty effective.
Depending on where the car was original sold and optioned, the floor may or may not have undercoating.
Honestly I wouldn't put any sticky type deadner in.
Only place I did that was the roof of my Grand? Wagoneer - and this week I was dealing with the consequenses. I did it to better insulate against the heat of the sun in the summer.
Maybe it helped a little but its a royal PIA when it comes time for repairs. I can't say I regret doing it but would not do it again.
Rust 'converters' are OK. The magnatite or whatever you want it is fairly stable but needs to sealed. It's similar to "bluing" processes except you can't use oil for the moisture proection.
It will need primer and top coats. Preferably a topcoat with very low moisture permeability.
Like RRR I've had pretty good experience with Rustoleum's rusty metal primer. Good in terms of doing what it says. The stupic aerosol cans always seem to clog up. I've pretty much given up on their spay cans.
I've also had good experience with Corroless primer (AFAIK not currently available in USA
)
The only 'one coat' rusty metal product I think is decent is Hammerite. it sometimes takes two coats and requires very good ventilation - the fumes are very strong. It's a styrenated something or another and generally nothing else can be painted over it at least for a few months.
That's what we thought but the first part can be tricky. It needs proper prep. Any crevices or rust bubbles that aren't busted down and cleaned away will eventually grow rust. Been there, done that.
That said, POR15 is a good epoxy paint with very low moisture permeability.
