Preparing the Metal
Remove any previous finish. If there is any finish left on the metal, it will interfere with the bluing process. This will result in an uneven and ineffective bluing. Grind and sand the piece until you have removed any and all finishes. You should see only bare metal.
Sand out any pits. If there are any pits in the metal, this is the time to deal with them. Use your sander or grinder to take the metal down around the pit. This will smooth the metal and give you a better finish.
Don gloves. Gloves will not only protect your hands, but they will also protect the metal from your hands. Your hands will leave oils on the metal that interfere with the bluing process. Wear nitrile or latex gloves to keep this oil from reaching the metal.
Clean the metal. Clean the metal with an alcohol wipe. You should then follow by cleaning it with a wax and grease remover (an organic solvent). Finally, use a soft cloth to dry the metal.
Cold Bluing
Apply bluing solution. You can buy bluing solutions premixed. Dip a cotton ball into the solution and apply it to the part evenly. If you are bluing a larger piece, use a rag or brush to apply the solution.
Wipe the metal dry. Once you have covered the piece with bluing solution, wipe it dry. Use a soft cloth, and make sure not to leave any solution in cracks or crevices. This will help ensure an even finish.
Polish the bluing. Use a fine grit sandpaper (for example 800 grit) to sand the finish. Sand slowly and carefully so that you do not go through the finish. This will smooth out your finish and make it shine.
Apply additional coats. Continue applying and polishing coats. When you have reached the desired color, you can polish one final time and then stop. Protect your finish by covering it in a protective oil (e.g. gun oil).
Hot Bluing
Scrub the piece in cold water. Submerge the piece in cold water and scrub with steel wool or a scuff pad. This will create small scratches for the bluing solution to adhere to. This should be done after any finish has been removed and the metal has been cleaned.
Immerse the piece in salt solution. A bluing solution is made of potassium nitrite and sodium hydroxide dissolved into water. The solution is heated to approximately 275 °F (135 °C). You should leave the part immersed for 15-30 minutes.
Rinse the piece in cold water. When you remove the piece from the mixture, put it immediately into cold water. This will cause the metal to contract and lock in the finish. Do not scrub the metal again.
Rinse the piece in boiling water. Boil plain water (with no salts). Dip the metal piece into the boiling water. This is the last step in the bluing process.
Apply water displacing oil. Removing the water from the metal is important. Put some water displacing oil onto a soft cloth and rub it into the metal. It will also leave a protective layer over the metal.
Bluing Other Ways
Rust blue antique parts. Rust bluing is the process of coating the metal in acid. Most commonly, nitric acid or hydrochloric acid is used. This allows the part to rust uniformly. Then, you scrub the rust off and repeat the process until the desired color is achieved. You can do this in a large container or vat that is tolerant to acid.
Fume blue for an even finish. Fume bluing is similar to rust bluing, but it creates a more uniform rusting. The part is placed in an airtight chamber and either nitric acid or hydrochloric acid (gas) is pumped into the chamber. The gas causes a uniform bluing. Any airtight container will work, as long as it cannot be corroded by the acid.
Try niter bluing. Niter bluing is the hottest form of bluing. In this case, you take a mixture of potassium nitrate salt and sodium hydroxide salt and melt them (700 °F (371 °C). The part will achieve a nice bluing when dipped into the solution for.
Contact a bluing expert. There are many types of bluing professionals. If you are not up to trying it yourself, you can reach out to a bluer to learn their specialty. Ask what kinds of bluing they do and what pieces they are willing to treat (e.g. large or small).
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