basicly you are removing the darkened color (blueing) for a more lighter color near to the white of steel. Very popular with gun engravers as it reveals fine engraving where it almost disappears on dark colors. Very attractive when used for highlighting and shading using different shades of color you can control. Bad news is that you are removing the rust preventing properties. Do not use on field guns...............Labrat
its been my experience that no matter which barrel finishing cold brown cold blue or cold blue rubbed back to the french grey color they all will flash rust if left in a moist environment like at tent or a gun sleeve check them all. wipe down with oil. run patch thru barrel the outside not really a big deal. it is this and handling the gun with dirty black powder hands, that after time gives that dirty nickel patina that i love on my guns
Phosphoric acid is what does the "French Gray" look and as stated it has somewhat of a rust inhibiting property but the good thing is if you get rust you can reapply the phosphoric acid and get the "French Gray" look back -- and keep it well oiled.
I stuck the breech end of a rusted up barrel in a bucket of Evaporust one time to try and get the rusted nipple out of a gun a friend didn't want. After about two days the nipple came right out but I was left with a half blued/half gray barrel. Didn't know the Evaporust didn't like bluing. I stuck the other end in the juice for another two days and had a grey barrel. Don't know what a French gray barrel looks like but this one was definitely gray. It took a browning solution good after that though
If it is on a used rifle/pistol, you can do this by scrubbing (erasing) with a rubber for suede shoes like this ...
If it is a new one, let it quiet, you may miss and in that case the jasper will not leave a clean gray...
Never do that on a barrel...