Thank you Gents for your replies.
I have also heard from a LAME that the quality of some early Cessna's alloy appears to be superior in some way that in his experience he's noticed less (and sometimes no) corrosion in those early models when compared with 70s and 80s examples. It was a "they don't build 'em like they used to" remark.
I wonder if others of you feel this way too.