while sand blasting the cab found a few rust holes.
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Depends. Them pinholes are only the worst of it... See, it rots from the inside out so it may look like small spots but the full extent will likely be much larger.

Ya gotta open 'er up to see the real damage and go from there. Don't be surprised if ya gotta fab and replace the entire flange.

 

dang, guess the 2wd non ac cab is looking like it might get a swap

 

Have you poked the area with an awl or screwdriver to find thinned areas? Ya might be able to get away sectioning out the bad area and then sealing the back side with POR-15 (or equivalent).