a lot of the rust these old cars obtain is rust from the inside not outside. yes rock chips the paint off that lower rear quarter, and will cause a rusty pinhole , but, it will rust from inside. cars like 68-70 b bodies that have a truck seal that NEVER ever kept water out of trunk, yes, that doesn't help., you will never find a rust free 68-70 b body that has seen rain. but these old cars will get dust inside that inside quarter, which turns to damp crap. like the inside of your doors, there is a drain there just like the rear qtr has that drain plug, but crap accumulates and doesn't get out.
take off the door panel; and see what is inside bottom of that door! same with that qtr if still in tack! crap that rusts it out.
like front frame rail under the floor. the rail rusts from inside not outside. the floor gets rust hole in it, water from there gets into frame rail along with dust/dirt. then it eventually rusts out.
after I replace that lower reat qtr and trunk extension, I spray epoxy primer/ paint inside best I can, then use a gun to spray undercoating/body Schultz down there. all I know to do.
main thing is after we restore a car, it generally is kept inside, hardly never gets out in the rain, so it should last many generations!????
