Bruce,
That is the beauty of phosphoric acid. After treatment the rust (iron oxide - Fe3O2) is converted to iron phosphate (Fe3PO4) coating. Multiple applications of phosphoric acid may be required to remove all rust and resultant black iron phosphate coting, but so far I have submerged everything for a few hours to expose shiny metal. It too has a powderish coating on it but not black as in the beginig stages of etching. The iron phosphate coating is what provides corrosion resistance but if you remove it you need to paint.
Muratic acid (hydrochloric acid) is bad about flash rusting metal quickly because it leaves no protective film behind.
Corey,
I have been through many a can of PB blaster and torch. Again thanks to the list for the trick.
Brett
