That insulation you’re talking about is an adhesive that is right at the body line. It holds the panel to the cab floor. I just did my corners as they had the exact same rust up high like yours. Yet the bottom where the drain is and even the inner section were good. Its weird how it happens. Here’s some pics to give you an idea of what it looks like inside
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That adhesive is right above that cut line. A little wire wheel and some por15 and primer and it was good to go. Of course I didn’t get pics. But here’s the final product also had to replace a section of rocker panel
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Don't mind the runs in the primer lol. All i had left was weld thru stuff at the moment and that stuff doesn't like going on too heavy
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