Tony, whether they are “brittle” or not depends at some level on how thick the galvanizing was applied. I have seen galvanizing that would sheet off in large flakes, sometimes all the way down to the bare steel. And in a case like that, the working of the joints as the car and roof were flexed would wear though the galvanizing.
I know you’re an expert in materials, but a metallurgist once told me that once rust starts, you’re screwed (his term) because even if you get most of it, even what appears to be ALL of it off, once it’s started you simply cannot stop it without extreme measures.
Schuyler Larrabee wrote:
This is certainly true, though galvanized coatings are not brittle. Note also, though, that galvanizing often fails at sharp corners, for example along the edge of the raised panels in a roof. Paint is thinner there. As Schuyler says, as soon as paint/car cement exposes the galvanized. it is at first protective (that's its purpose) but as more and more galvanized is exposed, eventually a pinhole begins to rust. All downhill from there <g>.