One of the really import things to understand when considering iron and steel is what rust is and how it works…a really simplified set of rules is that there are different types of rust to consider: There is stable rust, and unstable rust…your aim is prevent any rust ,or if you have rust to make sure it’s only stable rust.
 

Red rust..this is bad rust. It’s essentially active destruction and if not managed will destroy the iron/steel item. It’s Hydrated oxide Fe2O3•H2O (high oxygen/water exposure) with another contaminant such as salt. It is a sign that the environment itself is very corrosive, and is usually uniform….The classic example is an unprotected iron tool left in a garden shed.
 

Yellow rust. This is bad rust rust from iron oxides that is very soluble and is created in high water environments…a bike left out in the rain will form this yellow rust. Iron oxide-hydroxide FeO(OH)H2O (high moisture) It will run and will have run marks.
 

Brown rust..another bad rust. This forms in low moisture, high oxygen environments..Oxide Fe2O3 (high oxygen/low moisture). This is the sort of rust that an unprotected iron object will form in a family home. It tends to be spotty and will be propagated by contamination on the iron ( from a finger print etc).
 

Now we have good rust…BLACK rust…This is rust that forms in a low oxygen and low moisture environment, Iron (II)oxide – Fe3O4, also called magnetite.

It tends to take longer to form than other rusts and can actually form as a layer under other rusts, especially brown rust. Now the great thing about black rust is that it is stable and does not propagate. But more than that, it forms a layer that protects against active rust types ( red, yellow and brown)…for old artifacts made of  iron and steel ( swords and other weapons ) black rust can be a savior and preservative…very old European swords will often be covered in a coat of black rust and you do NOT EVER remove this black rust from these artifacts ( unless you are a philistine) as it’s the patina of age and authenticity and protection all in one. Japanese swords are a bit different in that the collectors aim to have the blade in polish and free of any rust ( to see and study the steel) but even Japanese swords preserve that black rust on the tang ( again to show the age )
The act of collecting and and appreciating Japanese swords is actually a bit of destructive process to gain access to the steel in the polished state  (no other area actively removes metal from artifacts…as a collector of old stuff it took me a while to get my head around the Japanese sword collection paradigm).

So with a blade that you are not going to likely get polished by a professional polisher, your best bet may be to follow a more European sword collecting approach which is to stabilize and protect only, until you can get an expert opinion on the worth of polishing.

So your aim is to stop the red, yellow, brown rust and stabilize to only having black rust. The way you do this, is to remove oxygen and moisture..so first oil…second oil and third oil. A good machine oil is best, such as sewing machine oil ( I use singers sewing machine oil. It’s cheap and easy to get hold of) If it’s heavily red rusted, I tend to actually give it thick layer of oil for a good few days…this really helps the oil penetrate the rust…carefully cleaning  with good quality lint free cotton cloth and 99% ethanol …then it’s a case of thin layers of oil and clean every few days. After a while you should have nothing left but black rust…then it’s just maintaining protection which is a very light ( no beading) layer of oil and  leave that black rust well alone unless your sending it to a polisher. Resist using acids and rust stabilizers on Japanese swords or any historic artifact for that matter
 

By Jonathan Davies