It's happened to all of us.  One day we pull out our favorite sword, knife, helmet, or other steel object and discover, to our horror, that it has rusted!  At least once a month those of us who frequent Internet bulletin boards dedicated to arms and armour see the panicked or dismayed posting of some poor soul to whom this has happened, and their plea: "What do I do now?  How do I fix it?!"

First of all, we need to understand what rust is.  While it may seem to behave like some villainous creeping creature, in reality rust is simply iron, the primary component in steel, mixed with oxygen, which is all around us in the air we breathe.  Rust is "iron oxide".  It's usually a powdery reddish-brown color, but can be black, brown, or even bright orange.

When a steel object, like a sword, for example, begins to rust, what happens is that the iron in the outer most layers of steel has bonded with oxygen from the air and changed to iron oxide.  The two pictures below show an unrusted object vs. a rusted one:

The first thing you will probably notice about your poor, rusted treasure is that the rust usually does not form as a nice, even coating over the entire surface.  As the image above shows, usually the rust forms in splotches - usually areas where by some mischance moisture was allowed to evaporate away on the exposed steel surface.  This moisture could come from something obvious, like drops of rain, or more subtle, like from fingerprints or even humidity right out of the air.  This means that some areas of the surface of your object are untouched, but others have begun to corrode. 

Even if the surface of your object is completely covered with rust, objects never corrode evenly.  The depths of penetration of the damage will vary, depending on everything from how clean the original surface was prior to the start of the corrosion to minute localized differences in the internal chemical structure of the steel.

Right about now you're thinking, "Look, I don't care how my treasure got rusted, I want to know how to fix it!"  I know, I know - but it's important to understand what has happened so that you can understand why there is only one option for "repairing" a rusted object.  Just a bit more introduction...

What makes things shiny

When you look at a beautiful, shiny sword blade or helmet, have you ever wondered why it is shiny?  What makes a mirror reflect?

Basically, the smoother the surface of an object is the more light it will reflect.  Conversely, the rougher a surface is the duller it will appear.  This is because a smooth surface reflects light uniformly, while a dull surface diffuses light.  Why?  Well, if you were to look at the surface of a dull object under high magnification, even one that looked or felt relatively smooth, you would notice that the surface is made up of many peaks and valleys.  The peaks and valleys all act like small mirrors themselves - and many of them direct the light away from your eyes.  This is why the object appears dull:

A smooth object reflects light well and appears shiny.

A rough object scatters light and appears dull.

Why rust makes things dull

Aside from the chemical changes that happen to the steel that make it less reflective, even if you could magically make all of the rust vanish from your rusted object it still would not be shiny!  Why?  Well as we noted above, when the surface of your object rusts first of all different parts of the original smooth surface are attacked.  These patches in turn corrode to different depths.  This has the effect of "roughing up" those patches of steel, in contrast to the shiny, smooth, uncorroded surrounding material.  In reality, corrosion removes material from the surface of your treasured object.  This is why rust so important to prevent - it is permanent damage to the item.

Repairing the damage

Like I said above - rust damage is permanent.  There really is no way to return the rusted object to its original condition - you can't microscopically fill in all of the rough spots created when the rust ate away the iron that was there.

The only option you have to restore a shine to an object that has rusted is to polish it.  Polishing obviously removes the rust, but it also has the effect of taking all of the mountains and valleys of the rough surface and smoothing them out - you in affect remove enough material until all that is left is a smooth uniform surface again:

It's important to realize that some objects just can't be repaired.  Sometimes grinding away more of the surface is more harmful than leaving things alone.  For example, SCA combat helms are required to be a certain thickness.  If one gets too aggressive when polishing a helm it is easily possible, especially with modern polishing equipment, to remove enough material to make the helm unsafe.  Other objects may have fine engraving on the surface, and polishing these items may obliterate the engraving.  This is why it is so important to protect objects from rust in the first place.

But let's suppose your object can be salvaged.  What is polishing?

What is polishing?

Quite simply, polishing is very controlled grinding.  A bench grinder or sand paper can be considered polishing media.  Very rough polishing media, of course, but polishing media nonetheless.  In order to get a mirror shine one works through progressively finer polishing media until the desired level of finish is achieved.  In fact, in professional armour shops the initial polishing can start with something as aggressive as a machinist's file, and end by using media as fine as baby powder.  This is the reason, incidentally, why mirror finishes usually cost more - they are labor intensive to produce.

Why does this matter?  Well the first thing that many folks do when they discover that their treasured object has rusted is they poke around in their garage until they turn up a piece of sandpaper or some steel wool, and they go at the rust spot with a vengeance.  To their dismay, they often find that the areas they have "polished" look worse than they did before they started!  This is because they chose a media that was too rough.  Or rather, they did not follow up with finer and finer polishing media until they matched to surrounding unrusted surfaces.  In fact, many people at this point simply "polish" the rest of the object at this point with their rough media, giving the whole thing a uniform, if somewhat duller, appearance.

Basically, if your object had a mirror finish before it rusted, chances are very good that you will not be able to restore the finish with any polishing media obtained at any regular hardware store.  It certainly is possible to achieve a mirror finish on the surface of an object completely by hand - you can work down through progressively finer grades of sandpaper and eventually down to cloth.  However, this is an extremely labor intensive way to polish an object and usually the very finest grades of sandpaper required are not available at your usual hardware store.

Mirror finishes are usually applied using specialized "buffing" machinery and specialized polishing wheels and powder.  If your object had a mirror finish and you would like to restore it, you are better off seeking professional assistance.

Why is a mirror finish good?

As I said, many people in a panic grab the steel wool or sandpaper, or chuck up the wire wheel in their drill and go at their object.  They might succeed at removing the rust, but they almost always end up with a satin finish on their object.  Many folks just "decide" at that point that they like the satin finish better.  Even if you really do prefer the satin finish look, there are good reasons for keeping a mirror finish on a steel object.

Remember our discussions earlier of what made something shiny?  A dull object has a rougher surface - many peaks and valleys.  All those peaks and valleys also mean that a rough object has a greater surface area  than a smooth object.  This means that a rough object has more area exposed to the air, which means it will be more prone to rust!  Worse, all those little valleys make for microscopic nooks and crannies for moisture to settle in and promote rust.  Generally speaking, the shinier an object is the more resistant to rust it will be.

Preventing rust

Hopefully you can now understand why rust is such a bad thing, and how important it is to keep it from happening in the first place.  The unfortunate news is that steel objects just plain require preventative maintenance.  There's no escaping that.  But if you've got a fine helm, sword, or other object that you want to keep looking its best, there are a few things you can do.  All of them involve some kind of coating on the surface of your object

Coatings

There are two basic kinds of coatings that can be applied to a steel surface.  Barrier and Sacrificial. 

Barrier coatings, as the name implies, form a barrier between the surface of the steel object and the atmosphere.  Paint, wax, or oil are all examples of barrier coatings.  The problem with barrier coatings is that they only work as long as they stay intact.  As soon as the coating is scratched or otherwise worn off, it stops protecting the metal.

Sacrificial coatings usually function as barrier coatings, but they go a step further.  Sacrificial coatings are made of a material that is "less noble" than the steel it is protecting.  The end result of this is that the base metal will not corrode until all of the sacrificial coating has corroded away first, even if the finish is scratched or worn to expose some of the base metal.  For a detailed explanation of sacrificial protection click here.  A common sacrificial coating is zinc.  Steel is commercially available with a zinc coating and is called "galvanized steel".  It is often available as wire or sheet metal.  Unfortunately, zinc coatings are not very shiny, and polishing is futile - though you may shine up the surface as soon as the zinc begins to corrode it will darken.  If you polish too aggressively you will remove the zinc and then you will be back at square one with bare steel again.

Paint is a good barrier coating, but obviously has the drawback that you won't see a nice, shiny steel object anymore.  One should resist the temptation to use clear paints, or lacquers, at least not without extensive testing to make sure they will work for your application.  The problem with these clear coatings is that they will eventually chip, scratch or otherwise wear away.  These exposed areas will then, of course, begin to rust.  The problem now becomes polishing.  Now in addition to polishing away the rusted exposed areas you will also likely find you have to polish away all of the old paint or lacquer.  Often times this can be more labor intensive than cleaning up a bare steel surface!

The best barrier coating for mild steel object like armour is, in my opinion, a light oil.  It has the advantage in that it can be easily removed when you want to actually polish the steel surface with a metal polish, and yet it can be easily applied or re-applied to the object as needed.  If you decide to use oil, make sure you get a good, quality, light oil.  Do not use heavy oils as they will trap dirt.  Do not use WD-40!  While WD-40 contains oils it also contains a lot of other stuff that will congeal and make a gummy mess.  The best place to get a good oil is at sporting goods stores or departments - usually in the firearms section.  Many oils are available in easy aerosol spray form and odor-free to boot.  You should make it a habit every time you touch or use a treasured steel object to give it a wipe-down with an oily cloth.  If you do this every time you are finished with your fine steel object you should be able to keep the dreaded rust monster at bay.

- Steven Sheldon, July 2002