One of the most common questions I get asked is, can I rust blue an old Winchester to the original recipe. To which the answer is always - "define Winchester's recipe". This questions is quickly followed by "why do you care about the recipe?".
Lets start with a bit of background on the Winchester rust bluing method. Winchester used rust bluing as the standard bluing process up until the late 1930's. In the early days, rust bluing was done by contractors who themselves employed various craftsman who each had their own technique and recipes for rust bluing. This resulted in a remarkably similar result, with only minor variation from firearm to firearm.
Later on, Winchester conducted the rust bluing themselves in what was known as the "Browning room", despite the fact almost all firearms were blued and not browned.
The point to make here is that like a timber stock, rust blued firearms coming out of Winchester were not exactly the same. So where does this variation come from?
The chemical recipe used to rust blue has two requirements. Firstly it should allow a controlled and dense corrosion to occur. Any chemical that induces rusting, could potentially be used as the rust blue solution, see my article What blue solution to use. Secondly it should occur quickly, the temperature/humidity chamber should also assist with this. The end result of this is your standard red rust, that is red oxide. The colour of which is entirely defined by the composition of the material and how well the material was prepared. So regardless of if you use salt water or some recipe made of various acids, red rust will look the same colour as long as it is applied to the same material. It may just not be as quick or as uniform. This red rust is then converted to magnetite with heat, once again will look the same colour regardless of the original rust inducing chemical used. Variation will only occur to uniformity, material preparation, the material itself and number of applications.
This comes back to what blue finish is. It is simply magnetite Fe3O4. In rust bluing it is "red oxide - rust" converted to "ferrous-ferric oxide - magnetite" using heat, commonly boiling water. Magnetite is by nature, blue-black in colour and its appearance is defined by its surface finish. A smooth sample will appear more towards the blue side, it will also appear glossier and generally more appealing, with variation of colour changing with angle of a light source and the viewers eye. A rough surface will appear black, with no gloss and no change of colour as light and eye angle changes.
So it is fairly easy to conclude that the surface preparation is fairly important to the end result.
As we have established, the variation to colour comes from the surface preparation, the material composition, uniformity and the number of applications.
Surface preparation - While mentioned previously that surface preparation is important, it is not as much as you might think when it comes to firearms. While it will make a huge difference at naturally occurring extremes of magnetite, rust bluing results in what is considered a satin finish, not gloss. Therefore the observed difference between preparing with 360 grit sandpaper and 600 grit sandpaper is negligible, if not unnoticeable. There is no point preparing a material to a smoothness that is smoother than the final outcome.
Material Composition - I have seen large variations in surface colour based on the composition of the base material. Some lower grade firearms will rust easily and only take about 3 applications to be fully saturated with colour. Better quality firearms, generally those made with 4140 steel take longer and generally result in a more appealing result. I have commonly found imperfections in material while rust bluing which can be seen by an area of a firearm not rusting as easily. These area are sometimes a challenge as the slower rusting area needs to match the colour of the faster rusting areas. Damascus steel is a great example of where the material composition affects the final colour of the rust blue finish.
Uniformity - Rust bluing uniformity is important, the application of the chemical formula and the use of a humidity chamber are important. However, as you apply application after application the less blued areas "catch up" with the more blued areas. Further to this, the more applications applied, the slower the colour builds. If enough applications are performed all areas of the firearm should be "saturated" to the point where the firearm looks uniform and with full depth of colour is achieved.
Number of applications - This is by far the most critical element of rust bluing, with each application the magnetite builds up on the previous application, the more applications the darker and deeper the blue. This will eventually saturate to the point where no more colour change is observed.
After considering all this it may be clear that as long as the colour is uniform and saturated there really is not much room to move on the actual colour itself. So the answer to "will it look that same as the original" is - yes it will look the same. I have blued many old Winchester's and compared them to perfectly stored original examples and I cannot tell the difference. There are usually other factors that would contribute far more to the look of the firearm such as existing pitting, bumps, file marks and worn lines.
Obviously the above does not just apply to Winchester but to any rust blued firearm.