Hocky1, you pretty much covered it. Generally speaking, you will not get corrosion on a panel that is sealed. It takes a breach in the surface, such as a stone chip or crack in the paint to start corrosion. The protective coating is gone and the steel is open to the elements.
At the risk of getting too technical, any type of crevice on a micro scale will concentrate the corrosive solution (or electrolyte) and bring on corrosion. This pretty much describes the vast majority of automotive corrosion. Filiform corrosion is a variation of this abovementioned phenomenon. It is corrosion that burrows like a mole under a coating. I don't know the reason why it acts in only one dimension.
Zinc coatings are only a stop gap measure... they don't last forever. Zinc sacrifices itself to protect the steel. When it's gone the steel begins rusting.
This is why it's so important to cover up any stone chips.
A good protective paint job on steel consists of the following:
- Zinc coat
- phosphate coat
- primer (e-coat is best)
- topcoat (colorcoat and clearcoat)
It is hard for repair jobs to equal factory paint jobs for a variety of reasons. Lack of phosphate and e-coat is some of the problem. Another is the lack of zinc coat from feather-grinding during body work.
95 E320 Cabriolet, 131K