Interesting points about salt in rusting solutions as well as neutralization methods. One of the uses for salt is when using Copper Sulphate as an etchant. The addition of an equal amount of salt to the solution (modified Bordeaux etch) prevents as much metal from being removed and makes the copper "felt" easier to remove, especially in less-than saturated strengths, say one-half ounce of each per gallon of water. That's the same strength as CuSO4 added to the logwood boil as per the Parker Process. In the P process it creates a fairly strong result, knocking the color back quite a bit, but really bringing out the contrast. I use it only for the first logwood boil, eliminating it for the second boil after two or three additional light rustings. If contrast is lacking, I'll use a quick dip in 3% Ferric Chloride.
For neutralization, besides the soak in cold caustic soda, in addition you can paint the tubes with a slurry of pickling lime and allow it to dry. After that a long soak in used motor oil (all the silicone has been burned out of it) and kerosine (50/50) really toughens up the finish.
