It should be noted for newbies not every FiST has rust issues.....nor does every FiST have paint chip problems. Many have had issues and I think the plant in Mexico had very bad QC in the paint section. Some unlucky folks have a poorly mixed brittle paint that chipped easily and some have a more resilient normal coat. Some got lucky and some didn't. 2 Fist's, 5 and 3 years old in our family.....no rust and 1 very tiny paint chip. We got lucky........
The Ford and other warranties cover corrosion on
panels and only if
perforated (not surface rust), same with extended warranties if they even address corrosion. In other words joints and seams where some owners are having problems are not covered. The makers have valid reasons for doing this on top of saving money, first is the obvious salted roads problem and another is modifications that increase vibrations and flexing of these joints, a lift kit in a pick-up was used as an example where I read this, or collateral damage from an accident. So in reality the makers seldom pay out on warranty coverage for "corrosion". You may find a dealer willing to try and fix it.
If you have rust in seams in my opinion it is a death sentence, rust is like a cancer, cut it out or it will continue to grow. That is the only reason I would ever sell my Fifi. If you see rust in seams and joints run like hell from that car or to your bank for money. Buying a used car without a very critical inspection has always been the same, caveat emptor.