I can totally understand the argument for not applying extra under seal over the factory installed stuff being a warranty issue, but I see no real reason myself, why it would not be acceptable to spray inject some fluid film or such like protection fluid INSIDE of the body shell box sections etc ?.
Treatment is always better applied to the inside, rather than the outside of any cavity.
Surface rust is never your problem, it penetrative rust that comes from the inside / out is the car killer !.
Consider just how much rain water
passes through the inside of the doors of your car in its life time.
That’s why they have drain holes fitted under neath.
Without them the doors would be full of water in a couple of months !.
Road dirt and mud trapped in pockets is a big cause of rust.
The dirt / mud remains almost constantly wet and rarely gets a chance to dry out.
It is in constant contact with the steel and the perforation rust monster gets to work.
If I am cleaning my car, I will jet wash under the wheel arches etc about once a month.
It’s amazing how much dirt is lodged under there !.
Even with the arch liners that they fitting these days.
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