re: Treating Metal Against Rust Using Tannic Acid and/or Phospheric Acid

Good write up Kelly.  I have used phosporic acid on a lot of vintage tool restorations.  It's simple to use and dispose of and the conversion process from iron oxide to iron phosphate is excellent corrosion protection.

I have stopped using it on anything that isn't painted though.  It leaves a really dull gray patina that I don't care for on things like hand planes.