Ebbie: Re post #22
I've seen wing spar corrosion in the fuel tank area a few times before on PA28s. Looks like galvanic action caused by the steel anchor nut may have set that off. All for want of a decent coat of primer or smear of jointing compound ...
I'm curious whether your previous few Annual Inspections had called for tank removal? Good inspection should have caught the corrosion in its early stage.