If you have read my instructions on wood finishing, you are familiar witht the concept of building up layers of oil by applying, rubbing off, and applying again until the depth of finish desired is obtained. The rust bluing process I will describe below is a similar methodology. One fairs the surface of the metal the same way we do wood (sanding), then apply a layer of rust (like oil on a stock it builds up a rough surface), then just like oil on wood we rub it off, and then do it again.
There is significant advantage to a hand rubbed oil finish. It has depth through the height of the deepest pores of the wood, but is not built up over the top of the wood. It is a rugged and durable finish that is easy to spot touch up. Similarly, a rust blue finish, while much more work than a chemical dip blue job, is much more durable and much easier to maintain than a chemical blue. A chemical blue as comes on our actions from the factory has no depth. It is a microscopically thin layer that provides zero protection of the steel from further rusting. It exists only for appearance. I have experienced, as I'm sure many of you have, coming home from hunting in wet conditions and have some clump of rust forming on the steel, maybe under the wood. This little clump of red rust will flick off easily with a finger nail; it has not had time to do anything at all injurious to the steel. However when picked off, the bluing comes off with it leaving bright steel behind.
A rust blued finish is created by rusting the steel then converting the red rust to black rust by boiling, and building up a layer of that rust through multiple applications. Later, when the gun is used in harsh conditions and some rust forms on the action, when removed it has zero effect on the finish. If anything, this new rust, once removed, makes that area even tougher in protection because you've basically just done another layer of rust as when the bluing was done to begin with.
Most bluing is not really blue. They are really more like 'black' finishes. Different steel chemistry affects the appearance of the blue more than the process of creating the blue. However, as a hand rubbed oil finish on wood looks different than a sprayed on Polyurethane, a rust blue finish on steel has a different appearance (most would think better) than a cheap chemical hot-dipped blue. Don't do this steel finishing 'bluing' technique thinking you are going to get 'blue'; it looks more like how an old well-used steel hand tool looks...that somewhat blackish kind of well oiled rust kind of look, but not even that so much, actually darker and richer looking because we are converting the red iron oxide to black oxide.
Sal Amoniac also known as Ammonium Chloride (buy on eBay. It's sold for tinning soldering iron tips)
instead of Sal Ammoniac Nitric Acid (paint store for concrete cleaner) can be used.
Carding wheel like this or oil free Libron Steel Wool
Or make your own de-greased steel wool: soak 0000 steel wool in lacquer thinner for an hour, then rinse with spray 'brake cleaner'
400-600 grit wet/dry sandpaper
Arm and Hammer Super Washing Soda
*optional Sodium Hydroxide (Lye)
*optional Hydrochloric Acid (Muriatic Acid)
Gallons of distilled water and a pot to boil it in
A gallon or more of used (or new) motor oil
Lengths of 2 1/2" PVC pipe, sliced 1/3 deep lengthwise with caps glued on the ends after slicing to create troughs. You will need one to pour boiling water in, one for soaking in a washing solution, and one to soak in motor oil when finished.
A cardboard, plywood, or styrofoam 'sweat box' with a light bulb for heat
A propane torch like for use with soldering copper pipe
Cotton or nitrile gloves
Long Rubber gloves for working with the cleaners
Arm and Hammer Washing Soda should be sufficient to allow the steel to sheet water. A more complete job however entails Lye to remove oils and Hydrochloric Acid (Muriatic Acid) to remove any organic material and rust.
Lye comes in some drain opener products like this
Hydrochloric Acid sources:
Muriatic Acid is sold as a pool cleaner and to remove excess mortar from bricks and can be found in the garden or pool area of any big box hardware store. It averages around 31-34% acid. It can be diluted in equal amounts with water to reduce it's strength for a bath or used in spots full strength if needed to clean an area that just doesn't seem to want to sheet water. Remember to add acid to water, not the other way around.
You may also use any of these common cleaners:
Lysol Toilet Bowl Cleaner contains 9.5 percent hydrochloric acid
Sno Drops Toilet Bowl Cleaner contains 15 percent hydrochloric acid
Lime-A-Way Toilet Bowl Cleaner contains 14.5 percent hydrochloric acid.
Do not leave Sal Ammoniac or Nitric Acid open in the shop. It will rust all steel anywhere in the building. I keep Sal Amoniac in a zip lock bag, and open the bag in the 'sweat box'. If using Nitric Acid, pour some Nitric Acid into an open container.
One does not even need to use a rust accelerant like Sal Ammoniac. In humid environments, one can hang the parts under the eaves and build a satisfactory layer of rust within a day.
Use gloves and eye protection when dealing with bases and acids in the cleaning and rusting process.
5. Allow the steel to form a thin even coat of rust.
6. Boil in distilled water to convert the red iron oxide rust to black oxide. If you want a browning instead of blueing, skip the boil.
Strip old bluing either with Naval Jelly or soak in white vinegar for 30 to 60 minutes, then rub lightly with 000 steel wool.
Degreasing is the key to success. The steel must perfectly sheet water which will only occur if there is zero grease in the steel.
Degrease with acetone and boil in a bath of Arm and Hammer Super Washing Soda. If there are stubborn areas that won't sheet water, try scrubbing that local area with a toilet bowl cleaner containing HCL (hydrochloric acid)
Dip parts in hot concentrated sodium hydroxide (lye) to saponify (to convert a fat into soap by treating with an alkali). After a water wash, a short dip in hot (not boiling) hydrochloric acid removes any inorganic surface contamination and rust.
An even coating of rust can be obtained by hanging the steel in a rust favorable environment. A sweat box can be made of wood, styrofoam, or even cardboard to hang the steel parts in, a light bulb for heat, and an open package of Sal Ammoniac, or a Sal Ammoniac and water solution, or nitric acid solution to provide the rust favorable environment.
Pour boiling distilled water over the parts in a PVC pipe trough to set the rust and turn it into black oxide. Card the rust off using oil free steel wool and/or oil free fine carding wheels and hand brushes made for the purpose. You may also rub the surface with blue jean material. In any case, made sure not to rub so vigorously that you remove the thin rust layer. You only want to remove scale.