Stormin Mormon soaked rusty pliers in HCl. Rbowman recommended
phosphoric acid, sold to etch concrete. That reminded me of naval
jelly. I knew it contained phosphoric acid. It also contains a gel and
rubbing alcohol. I wonder what the rubbing alcohol does.
I've had some success with vinegar and with vinegar and salt. Now I've
read the proper way to use vinegar and salt. Use an inert container
such as plastic or glass, big enough for the rusty item. Pour in enough
vinegar to cover the item. Stir in an excess of salt. Put an inert
spacer on the bottom to keep the steel from contacting the salt. Let
the item soak, completely submerged.
Vinegar works, but this mixture also contains HCl. The acetate
apparently keeps the HCl from eating the steel. You can pull the item
out to inspect and brush. When it's ready, immediately wash and oil.
Phosphoric acid is sold for converting rust (not removing it). I don't
know the concentration of the etching stuff, but Ospho is 75%. (Naval
jelly is 25-30%.) You brush or spray Ospho on and let it dry. If the
surface is badly pitted, you may repeat. If you brush away any white
powder, you have a good base to paint.
Ospho says their product can be used if you want to repaint spots where
rust is coming through a painted surface, but you should test to see if
Ospho damages your paint.
I'm looking forward to trying the proper method of using vinegar and salt.