Half the elevation has surface rust and a couple of spots where it looks worse, but replacing the lot is a big spend we'd rather avoid. How do I decide whether coating buys us real time or whether we're just papering over a sheet that's on its way out?
The deciding factor is how much sound steel is left, not how much rust you can see. Where corrosion is confined to the surface and the sheet still has its structural thickness and integrity, a proper coating system can arrest it and add many years of life at a fraction of replacement cost. Once you have perforation, significant section loss or failed insulation behind the sheet, coating over it only hides a problem that will keep moving. The honest call needs the metal inspected in a few representative places, because two elevations that look similar can be very different underneath. A liquid-applied Overcoat system is often the sensible route for sound sheets, but get it surveyed first so you are treating, not masking.
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