This is a roof repair I have done. The outside had half dozen or so pinholes.. This was the inside..
Given your circumstances, you will likely have similar, and when you weld you will find the thin spots and also find it challenging to weld up due to blowing holes from the thin spots that are hiding. Also, to speak to how dry you plan on keeping it, when you start welding in the vicinity of a deep, scaly pit that is hiding on the back side, the weld activity will awaken such pits that you'll see them again within 2 years, dry or not. It is cheaper to fix now before wasting paint on something that is going to rust through..
Here is a thread on the entire repair. Note the dark grey circles in vicinity of the visible pin holes. This is a tell tale sign of a deep pit getting ready to break through in short order.
https://www.trifivechevys.com/showth...f-rust-repairs
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