Just wanted to add my own experience with respect to this issue. Have a low mile 2017 Fiesta ST and discovered hem rust under the hood (on both sides) when detailing the car. I searched this website and came across this post that pointed out other problem areas, and sure enough, I discovered hem rust on my rear hatch and 3 of the doors. My car is still under warranty until October this year, so I made an appointment with my Ford dealer (they have an in-house body shop) to get this documented and submitted for warranty repair/replace. Long story short, Ford replaced the hood, hatch, and 3 doors. They also paid for my car rental (Ford F150 crew cab with 5.0) for the week and a half that the repair took. Was really impressed that they didn't fight this at all given the significant costs involved. The paint match was perfect too.
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