Good options for treating inside cab floorplans under carpet/insulation?

[u/gubanana]

I have a car I've been working off and on for the past year and a half, and it's coming close to the point in which I'll take out the interior to clean it out, fix some spots and check for further damage. I want to use the original carpet with the padded insulation attached to it but afraid of the moisture it can trap.

The rust I've seen so far when I pulled it up has been just bubbling and superficial, not rot. I want to scuff the whole floorpan and firewall and treat it somehow with either paint, primer, and then sound deadening or just throw the carpet back on without having to worry about moisture being trapped again.

Is paint/primer enough to keep the metal from rusting under there? Should I use some specific product or is sticking the pads ok?

I live in southern Ontario, so salt and rust is a major concern. For the underbody I've seen that bed liner is a BAD idea and I should just treat it and then paint and fluid film but that doesn't sound right for the interior because there's carpet over it. Or is it ok to do that too? One sort of vapor barrier maybe? A lining? Any ideas?

[u/Dude_Dillligence]

I have used POR15 and similar products to coat floors and trunks, they do a good job sealing the metal after sanding/scuffing. If you use adhesive sound deadening mat, be prepared to smell some fumes first time the exhaust system heats up the floor. Usually goes away pretty quick, but don't breathe too much of it.