Alan - I was doing final prep for paint today and ran across a place that got me thinking about this - where I'd drilled painted steel panels last summer but didn't rivet them, there was rust around the holes ... on the same panel where the top edge had been riveted immediately, no problem - I think maybe with the trim rivets, because there was no treatment after installation, and because pop rivets never really seal to the panel 100%, air and moisture [even just high humidity] can get at where the rivet jams against the unpainted steel, any place moisture can get at the metal it'll work out under the paint
my "forensic analysis" of the original construction of this bus indicates the panels were coated with zinc chromate primer [steel as well as aluminum] before assembly, and then after the panels were riveted on the whole thing was primered [red oxide] before paint ... pretty good seal against any moisture/oxygen getting to the metal ... I'm not doing quite as much, but new metal is being painted before assembly, and everything is getting a coat of DP40 epoxy now [steel and aluminum] before color
Bill
