Footwell corrosion

This is something l don’t yet have on either of my TD5’s
Although there is surface rust behind the carpets on my 110.

Where should l apply rustproofing spray?
Do l apply it behind the carpets in the footwells
Or is it underneath the vehicle that l need to apply it. Is it a single skin?

Apply above the bulk head out riggers. Mud seems to build up there.
Also by the heater matrix bracket. Holds water
Yet to been single skin.

Thanks

The area above the front outriggers gets regularly cleaned of any mud, and sprayed with rust proofer.
I use PMA spray grease as it forms a coating that stays sticky but doesn’t wash off, and it doesn’t build up layers of useless underseal from repeated spraying.