One GREAT rust-removal method I learned about on a muzzleloading-gun forum--and then tried it on a gun with some parts rusted into immobility, and it worked PERFECTLY where penetrating oil and other methods did not: electrolysis.
This is really easy. I got an electrically-non-conductive basin, like something from Tupperware or the Wal-Mart equivalent. Got some washing soda from the laundry section of my local supermarket. Mixed up a solution of washing soda in water, maybe a teaspoon or tablespoon per quart. Stirred until the washing soda dissolved. Y
Borrowed a battery charger, the kind used for cars. I set up the plastic bin outside, with enough washing-soda solution to cover the gun barrel I was de-rusting. Wrapped a wire around the breech/rear end of the barrel so that the wire stuck securely up out of the solution. Secured it to the side of the plastic bin with duct tape. About a foot away, or so, I took a large,steel bolt that I didn't mind rusting (because it WILL rust, badly and fast), stuck one end of it into the solution, the other end sticking out of the solution. It is important to make sure that the bolt (or other to-be-sacrificed metal item) and the part being de-rusted do not touch, and will not touch even if the tape setup weakens over several hours.
I am told, by the way, that one must not use stainless steel anywhere in this equation, or it will release toxic compounds called chromates, which you do not want.
I then hooked up the NEGATIVE clip of the battery charger to the wire that was wrapped around the gun barrel (my mnemonic: some people are very negative about guns); I hooked the POSITIVE clip to the bolt; and I powered up.
Some people (maybe depending on electrical charge) report that the immersed metal begins bubbling instantly, they say releasing oxygen from one of the metal pieces and hydrogen from the other. In my case, I saw no immediate result, but went back an hour later and found the bolt and the gun barrel both releasing bubbles, the rust on the gun barrel turning into a black paste that I could smear off, and the bolt covered with a huge amount of multi-colored, foaming rust. A few more hours of that, and a cleanout screw and percussion-cap nipple that I'd been unable to remove with any manner of wrench use or penetrating oil screwed out nicely. The barrel ended up looking essentially NEW, like a right-out-of-the-box unblued kit barrel, except for a few places where rust had made pits in the surface. The rust was gone, though the pits remained.
The real beauty of this was that it removed little, and maybe none, of the good metal. It selectively targeted the rust, I believe. I imagine it'd also remove rust-like finishes like bluing or browning, but you can always reapply those. It seems to me potentially a very good method of removing serious rust from any useful or historically-interesting iron or steel item, as long as you don't mind REALLY removing the rust and patina.
It may help to immerse only part of your knife, by the way, depending on what your handle and other parts are made of, and whether or not you want to risk having them in the solution.
I am a complete ignoramus in dealing with electricity, by the way, so I kept myself and kids away from all parts of this setup while the power was on. I don't really have any experience, nor knowledge, of hazards involved. But I've got to say it worked!
There are very good discussions of this method, including pictures, over on muzzleloadingforum dot com .
Have fun!
