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Introduction
Any shooter and gun collector knows that gun metal needs to be protected from corrosion.  Moisture from the air, moisture and oils from our hands, even salt in the air if we live close to the ocean can all contribute to the formation of Fe₂O₃ iron-oxide, otherwise known as rust.  Over the decades firearms manufacturers have devised various methods to protect the metal.  Today, the most common protection is called bluing. 

From Wikipedia; Bluing is a passivation process in which steel is partially protected against rust, and is named after the blue-black appearance of the resulting protective finish.  True gun bluing is an electrochemical conversion coating resulting from an oxidizing chemical reaction with iron on the surface selectively forming magnetite (Fe₃O₄), the black oxide of iron, which occupies the same volume as metallic iron.  Black oxide provides minimal protection against corrosion, unless also treated with a water-displacing oil to reduce wetting and galvanic action.

In contrast, rust, the red oxide of iron (Fe₂O₃), does not occupy the same volume as iron, thereby causing the typical reddish rusting away of iron.  Both "cold" and "hot" oxidizing processes are called bluing, but only the "hot" process provides any significant rust and corrosion resistance, and then only when also treated with an oiled coating.  This explains why blued firearms must also be oiled, and why surface rust appears on blued firearms that are not kept oiled. 

Complete restoration of a firearm would involve hot bluing, something that I, as a home hobbyist gunsmith, am not set up to perform.  However, for touching-up exposed areas, or bare metal exposed from milling, filing, or cutting, I use cold blue.

There are a number of products on the market for cold bluing.  The products I use most are shown in the above photo. 

Degrease
Before bluing a part it must be thoroughly degreased, otherwise the bluing won’t take.  I use acetone and a Q-tip to degrease the area to be blued.  If I have to touch the degreased area with my hands, I wear rubber or vinyl gloves to prevent moisture and oils from my hands getting on the bare metal.  If the metal is not properly degreased the bluing will appear splotchy or uneven.  Properly degreasing the part is vitally important.

Shooter Solutions Rugged Gun Blue

This has to be the best solution (pun intended) for cold bluing that I have found.  The results are very close to commercial hot bluing.  This is my go-to cold blue product from now on, for both touch-up and large items!  This product is an acid so gloves must be worn at all times to protect skin from contact.  The instructions are somewhat difficult to read, but the process is easy:

1. Bead/sand blast or sand if necessary
2. Degrease – I use acetone
3. Dip or soak the part until it is dark blue/black – properly prepared metal only requires a few seconds, contaminated metal may take longer
4. Rinse the part with water to remove the excess acid solution
5. Dry with a hair dryer, heat gun, or air compressor
6. Apply gun oil

I used this product to finish a 1911 frame, refer to my article Building a Dedicated 1911 .22 LR Pistol.

Van’s Instant Gun Blue
This is the best, and probably most difficult product to use.  It really isn’t very “instant”!  This product produces the closest result to hot bluing and I’ve discovered it is the best product to use for bluing larger areas.  For best results the part must be thoroughly degreased, heated with a hair dryer, then immersed in the bluing solution for 5-10 minutes.  For small areas the manufacturer recommends using a clean, new toothbrush to brush the solution on the area to be blued; the secret here is to keep the area wet and have patience; remember, 5-10 minutes to get a deep, even blue.

A word of caution here; immersion works well if there is no existing bluing on the part.  If there is existing bluing on the part, rust particles form causing the solution to turn orange.  These rust particles can attack the unblued part and cause pitting.  I know this because I was rebluing a receiver that was already partially blued, and when I immersed it, rust particles pitted my newly polished receiver!  I had to draw-file and repolish the receiver, then used the toothbrush method to blue the exposed metal.  The rust particles can be filtered by straining the remaining solution through a coffee filter.

I completely reblued the receiver for a Ted Williams model 100 .30-30 using Van’s and the result was stunning.  However, I had removed all of the old bluing and totally immersed the receiver.  Refer to my article Restoring a Ted Williams Model 100 .30-30.

Brownells Oxpho-Blue®
Oxpho-Blue® is manufactured by Brownells.  I use this product for touching up small areas.  Once the area is degreased, I use a Q-tip to apply the solution and the area immediately turns dark blue-black in color.

Birchwood Casey® Perma Blu® Paste and Liquid
Birchwood Casey® makes their excellent Perma Blu® product in both liquid and paste.  I prefer the paste because I can apply it with a Q-tip and it stays in place, whereas with the liquid I have to continuously keep rubbing the area.  I leave the paste in place for 3-5 minutes in order to get a deep blue-black color.  I probably use the paste more often than any other product.

Birchwood Casey® Aluminum Black™
Birchwood Casey® also produces a product for touching up aluminum.  I have used this product with pretty good success for covering up dings and scratches.  Again, the part must be thoroughly degreased, but I use a Q-tip to apply the solution and the exposed aluminum turns black after setting for 1-2 minutes.

Summary
Any time I expose bare metal I use cold bluing to protect that exposed area.  This is especially important when polishing the internal area of a blued firearm.  Polishing removes metal, and in so doing, also removes the protective bluing.  These polished areas need to be re-blued to provide some protection, and cold bluing is the least expensive means of providing that protection.  Cold bluing also can enhance the appearance of an older firearm by touching up dings and scratches.