Cold bluing a gun barrel and parts at home

Cold bluing restores a dark protective and decorative finish on metal without heat; to cold blue a gun barrel at home, use cold bluing fluid. Sand and degrease the damaged area, apply the fluid in thin coats until the colour is dark enough, rinse with water, then oil and polish the area. The fluid suits steel, except stainless, as well as brass, copper, silver and bronze.

What you need

Cold bluing fluid, 40 ml

Deep black finish

Gun oil, 210 ml

Fortified formula

What cold bluing is and where it is used

NANOPROTECH Cold bluing fluid restores the natural black colour to worn spots and scratches on the outer surfaces of shotgun, rifle and pistol barrels and knives, without heating or a trip to a workshop. The fluid is an aqueous solution of selenium and copper salts with mineral acids. For knives, brass and bronze, see bluing a knife and metal at home.

Cold bluing a gun barrel: step by step

1. Prepare the gun and yourself. Make sure the gun is unloaded. Put on gloves and safety glasses.
2. Shake the bottle. Shake the fluid well before use.
3. Sand the damaged area. Use 1000-grit abrasive paper or an abrasive pad to get down to clean metal, removing old bluing and rust. Do not use rust converter for this.
4. Clean and degrease. Use a degreaser that evaporates without leaving a film, and do not touch the area with your fingers again: on greasy metal the bluing comes out patchy.
5. Apply a thin coat. With a fine brush or cotton bud, apply the fluid only to the repair area and leave it to dry for 5–10 minutes. Keep it out of the bore, gaps and the soldered joints of the barrels: acid cannot be rinsed out of them. Never dip a brush that has touched the metal back into the bottle.
6. Rinse and dry. Rinse the area with water, which removes leftover acid and salts, and wipe it dry at once. There is no need to heat the part.
7. Repeat until the colour is right. Repeat the coat, drying and rinsing while the colour keeps getting darker.
8. Protect the finish with oil. Coat the treated area with NANOPROTECH Gun oil and polish it. The oil's nanocoating sets in 10 minutes; wipe off any excess if needed. Maximum protection forms after 24 hours — until then, do not put the item in a safe or a case with other guns.

For further barrel care, see how to clean a gun barrel and preserve a gun.

Why it may not work

• The metal is unsuitable. The fluid does not blacken stainless steel, gives a weaker colour on high-alloy steels and is not used on aluminium and its alloys or on chrome- and nickel-plated parts. Some airgun parts are made of light alloys, so check that the part is steel.
• Poor preparation. An unshaken bottle, or old finish, rust or grease left on the metal, including from fingers, gives a pale or patchy colour.
• The wrong coating routine. A thick coat or a longer dwell does not help. If another thin coat no longer darkens the colour, go back to preparation.
• No rinsing or oil protection. Freshly blued metal can then rust. A lubricant for moving parts does not replace gun oil.

Safety precautions

Because of the mineral acids, the fluid is toxic, harmful if swallowed, causes burns and may damage the unborn child. Wear gloves and eye protection, work in a ventilated room and do not inhale the vapours. Do not heat the fluid or immerse parts in it. If it gets into the eyes, rinse at once with plenty of water and see a doctor; see a doctor at once if you feel unwell. Keep the bottle locked up, out of the reach of children, and do not pour the fluid down the drain. Do not use it on kitchen knives, items that touch food, wheel bolts, brake parts or other safety-critical fasteners. Gun oil is a flammable aerosol: do not heat the can above +50 °C or spray it near flames.

Frequently asked questions