4.5 in disc grinder down the side edge with some 36 grit. you aren't reusuing it, so it's a matter of what gets it off quick and easy without tearing up the inner skin.
use something (spot weld cutter, zizz wheel) to take the couple tacks off the flanges, grind the welded areas around the hinges. once you grind the edges off you can just peel back whats left.
besure to prime the flanges and inside of the inner skin before you put your new skin on. weld through primer is choice for flange areas.hth
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