You live in a damp climate like me..
I have owned 4 x punto 199, and they all dribble water from the tailgate when opened..

(After rainy days..)
I have not investigated.. But also not seen rust..
TBH some earlier Fiat's I owned had composite tailgate.. So in my mind I had not been concerned.
My mid 80's cheap cars All rusted,

I've not addressed rust since the 1989 tipo arrived
@PuntoHowTo seems to be knowledgeable on "Germanic" corrosion treatment products, may be pretty relevant to you
