How to use cold gun blue (and why it isn't working)

Cold blue is a quick way to touch up worn or scratched bluing on a gun. When it seems to do nothing, it is almost always down to one of a few simple things. This guide covers why it might not be working, and how to get a good, even finish.

Why isn't my cold blue working?

The usual causes, most likely first:

1. Oil or grease on the metal. This is the most common cause by far. Even a fingerprint will stop the blue taking.
2. The wrong metal. Cold blue only works on carbon steel. It will not work on:
   • stainless steel
   • aluminium or alloy (many shotgun actions and receivers are alloy)
   • parts that still have a modern factory finish on them, such as nitride, Cerakote or parkerising.
3. The metal is too cold. In a cold garage or workshop the reaction slows right down.
4. The bottle is old or contaminated. This can happen if a used applicator has been dipped back into the bottle.

If your part is an alloy action, you need an aluminium black product instead of cold blue.

How to use cold blue

1. Strip back to bright, bare steel. Use fine wire wool (0000) or wet-and-dry paper. Any old finish or rust has to go.
2. Degrease thoroughly. Use acetone, brake cleaner or methylated spirits. After that, don't touch the area with bare fingers; disposable gloves are ideal.
3. Warm the part slightly. A hairdryer is enough. It should be warm to the touch, not hot.
4. Pour a little blue into a separate dish. Never dip into the bottle. Apply it with a clean cotton bud or pad and keep the surface wet for 30–60 seconds. You should see it darken quickly.
5. Rinse with cold water. This stops the reaction. Then dry the part.
6. Rub back lightly with 0000 wire wool and repeat. It usually takes 3–6 coats to build a good, even colour. One coat on its own will look thin and patchy.
7. Oil well and leave it overnight.

Cold blue is a cosmetic touch-up rather than a hard-wearing finish, so keep the area oiled and expect to touch it up again from time to time.

Safety

Cold blue solutions are acidic and toxic. Wear gloves and eye protection, work somewhere well ventilated, keep it away from children, pets and food, and read the safety information on the bottle before use.

Still stuck?

Give us a call on 01786 450404 and we'll be happy to help.