I have a German fixed blade knife that was manufactured during or soon after WWII. It was purchased from a knife dealer in the late 50s. It was purchased in a seedy area of Seattle (an important port) down by the docks and it says “SCHNEIDTEUFEL SOLINGEN on it. It’s a fixed blade knife with a blade about 131mm long and what I believe to be a real stag’s horn handle. I don’t know enough to say this with certainty, but it’s probably a type of Bowie-style knife. It’s quite handsome and it brings back good memories so I would like to try to restore it. (It’s rather badly tarnished but nothing is the color of rust and I would like to remove the tarnish. I don’t know if it would be possible but if I could remove what appears to be a plug at the back end, I might even be able to tighten up the handle a little as well.) I do remember that when I received it, it had a thin coat of protective oil on it. By the way, I have the original leather (probably rawhide) sheath but I can’t yet find the felt-lined wood presentation box that came with it.
I should also mention that I’m trying to assemble a diverse collection of knives – different styles, from different eras, folding and fixed blade, etc. As an example, my next purchase will probably be a collar locking folder with a real wood handle.
I already restored a lot of straight razors to use them. There is several “ways” to restore a blade.
Some people like to keep the patina that contributes to the “history” and the value of the blade. They only remove rust, but is is hard to remove rust while trying to keep the patina.
The less agressive way of restoring tries to avoid to use sand-papers because it will modify the blade. For exemple if there is traces of the stone wheel that was used to grind the profile of the blade, you’re going to remove it. Bad idea. In europe and probably in countries with long history we generally like to restore that way, and people in other countries don’t understand why, I had “fights” on straight razors forums to explain this point of view. I know, this is only a matter of taste.
Rust and patina can be removed, without modifying the blade, with compounds and a dremel. I use dialux. Not the orange one because it is too agressive. The green one is less agressive but you have to use it lightly to not remove the “manufaturing grinding traces” I talked above. To have a high shinning blade but with a non agressive compound, the blue dialux is perfect.
It is very very long by hand. A dremel is faster, and a “buffer” is magic. By hand, you can use the green dialux.
I know that there is a lot of people in USA that use “maas” to remove oxidation. Never tested, during a long time it was hard to find and very expensive to import it in europe, but it seems to be effective.
The thing to know is that when you remove rust, there will be some pittings. If you decide to keep those pittings and not remove it with sand paper, rust risks to reappear quicker in those areas. Your blade will need more care (oil it, keep an eye on it, …) to avoid this.
I would not remove the patina, you knife is acquiring a natural blue similar to the finish applied to firearms. Shiny white steel, unless kept regularly polished, will tarnish quite quickly and possibly rust as well. The blue inhibits this.
I have cleaned up quite a few old knives and tools picked up cheap from garage sales and flea markets. By far the best thing I have found so far is a wire cup brush wheel on a 4” angle grinder. It removes rust but as long as you are not too heavy handed with it, it leaves the patina/blue pretty much intact.
Be warned, this requires a very good grip on the knife/tool. Clamp the knife well. The wire wheel can fling a knife a long way. Work with the brush spinning from the flat towards the edge, so you don’t catch the edge of the knife with the brush. Use a very light touch only, with the wire barely touching the blade.
I found linseed oil the best thing for protecting blades for storage. It forms a semi plastic coating that is very protective, and is easily removed with a brief application of the wire wheel if required.
Deep rust pits, as stated earlier, will inevitably produce more live rust. I don’t know a way to prevent this without affecting the blade either by grinding it off or using a chemical treatment like phosphoric acid. Neither is recommended on good knives.
Any kind of restoration lowers a knifes value, as does sharpening.