Phosphoric acid is pretty tame as acids go. I would use it full strength. I wouldn't trust the residue at the bottom of the pits to be "inert". If it were my truck I would do as you have, use the acid full strength, then I would treat with a rust converter to treat what's at the bottom of the pits. Don't ever let bare metal sit over night or you'll get what you have with your second pic (or worse). Depending on the humidity it can only take minutes for rust to start forming on unprotected steel.
Save a life, adopt a senior shelter pet.
The three main causes of blindness: Cataracts, Politics, Religion.
Name your dog Naked so you can walk Naked in the park.
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