Etching long blades is really a problem, specially if you cant unmount the hilt.
(For "newbies": Do NOT touch a hilt from organic material with acid!)
For knifes i use a big jar were i can bleach all the blade and feel safe about the hilt. This way a solution 1/3 of vinegar for few hours gave me exelent results. Maybe a different solution or time could do something better. I dont know.
I prefer vinegar for 2 main reasons:
1. It is cheap and easy to find
2. In my country we produce a lot and it is "traditional" to use vinegar for thousand of actions, among them to clean things (my wife use it for cleaning the carpets
A friend sugest that for a long blade we have to make a "bath" from adhesive tape like this: [_]
are the sides of the tape and _
is the blade. I have not tried it yet but it looks ok and I will do it.