I did another thing.
Cut out and replaced the offending inner structure.
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Replacement out patch made piece, by piece, by piece. I mentioned this before: TIG over MIG. Beats grinding. However it does cause the part to shrink... fortunately it shrunk in my favour.
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Makes for a far less sensitive joint to grinding through than if you just weld and dress the outside corner.
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And... done. The keen eyed amongst the two people watching this thread will see a 1x1 patch on the outer lip. That was cut out to access the rot behind it which needed a 3/16" plug to fix the inner lip, and then fix the rest of the offending area.
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One more section to go!