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.
The finish, blue, black or somewhere in between depends on the quality of the steel
The polish of the steel before you start rusting determines the appearance when finished.
Once the steel has been de-greased, do not touch with bare skin AT ALL until the entire job is finished. Do not card with steel wool or wire wheels that have ANY oil or grease on them.
While used motor oil is certainly the cheapest oil, new oil imparts a brighter look to the finished steel surface than what the dark colored used oil has.
1. Strip old bluing.
2. Polish the steel with wet/dry sandpaper.
3. chemically strip oil from the steel.
4. Plug openings if desired or wipe down with mineral spirits to prevent rusting.
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.
7. Card the rust off
8. Repeat steps 5, 6 and 7 maybe half a dozen or ten times until you either it stops taking rust (how good a finish is that when it doesn't want to rust in a pefectly rusting environment) or you get the appearance and level of protection you desire
9. After final carding, soak steel in used motor oil or dewatering oil for at least several days. Slight warming of the oil can help get it soaked into the rust finish.
Strip old bluing either with Naval Jelly or soak in white vinegar for 30 to 60 minutes, then rub lightly with 000 steel wool.
Polish the steel
You do not have to go any finer than 400 grit to get fine results. 600 grit provides a more polished surface, and 800 grit might be going to high a polish but often us airgunners like the high polish. Too high a polish seems to inhibit rusting from occuring evenly as if the fine scratches give the rust a place to build on. A high polished appearance will appear as the rusting process progresses. Finish with a light pass with a fine wire wheel and oil. This will burnish any remaining grit marks while maintaining sharp edges and lettering. Use a water (whet) stone or wet/dry sandpaper on an aluminum block on flat areas to maintain flats. If you choose to use buffing compound on a wheel to polish the steel, you will have to use a solvent to remove all traces of the compound.
Plug any openings if you wish
Wipe any interior surfaces you don't want to rust with mineral spirits and plug any openings with dowels or bolts. However I and many others feel it's not only fine to rust blue all surfaces (it is after all affecting a microscopic thickness of the steel's surface), but advantageous for the protection of the gun to do so. That just means running a de-greased brass brush down the bore and a light flapper sander down compression tubes during the carding off step. Make a light flapper sander with a dowel, slit one end to slip a strip of wet/dry sandpaper (garnet paper can have additives) and use in a drill.
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)
For more a more complete job:
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.
In either washing case, the easy or the harder, a second rinse with hot distilled water followed by drying with a hot air blower finishes the prep.
You will have to clean the steel more than once. The cleaned surfaces must be completely wettable in the distilled water step; if the last time you rinse the part it is not covered by an absolutely continuous film of water it is not clean yet, and the rust will not cover the steel evenly in those areas. Note any areas that the water does not sheet but pulls away from the steel and re-clean those areas.
After the degreasing, boil the metal in distilled water and allow to cool thoroughly.
After de-greasing, do not touch the steel with skin, no matter how clean you think you are. Cotton or nitrile gloves only.
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.
The rate of rust production will vary based on weather conditions, but certainly less than 24 hours, and more likely only two or three hours. Do not let the rust go any further than the creation of a thin even coat or you risk having it pit the steel.
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.
Repeat the rusting, boiling, carding maybe half a dozen or ten times until either it stops taking rust (it's a good protective finish if it doesn't want to rust even un-oiled in a pefectly rusting environment) or you get the appearance and level of protection you desire.
After the last rusting, boil in a Washing Soda solution to stop the rusting, then do a final carding. Heat the metal by playing a soft propane flame over the steel to drive out any residual moisture, then soak in a warm motor oil bath for several days. PVC pipe sections are an idea for a bath that is conservative with the oil. Use used oil for cost savings and a slightly darker color of the finish. Use new motor oil for a slightly brighter coloration.