Can you give us the details of this method?
I always recommend a bluing cabinet because you can count on a consistent and repeatable atmosphere in which to learn the best tweaks to your method of rust bluing for best results. It is also able to be used the year around not just in the humid months which is when Neidner did their rust bluing.
The Neidner method starts with mixing the solution which is comprised of Nitric, Hydrochloric acid, black wire and distilled water. The solution when mixed has a very vigorous chemical reaction involving a lot of heat and poisonous fume release so must be mixed in a pyrex glass container, out doors, if done on a personal level. I've mixed my own twice and have survived with no issues but there are some other solutions out there that I've read work well without one having to mix the chemicals personally.
The regimen is done in 3 hour sessions with a bluing cabinet then carded with wire wheels and brushes followed by 10 minutes of boiling then repeated. I usually use 6 applications.
When a barrel steel is found to be resistant to the solution a 1 to 10 ratio of distilled water and nitric acid can be applied as an etching agent or you can sand blast followed by a glass bead blast which is what I prefer.I have found 70 grit garnet sand followed by glass beads give a beautiful satin finish with very deep penetration of oxides for a very attractive and durable finish that wears extremely well over a long time.
The big thing I have found with rust bluing is to not over apply the coats. The first application of solution in each evolution needs to be done evenly with no drips or runs. If you miss a bit because your pad or swab ran dry leave it until the next evolution. It is hard as can be not to try and go over it to get the area wetted the same but leave it alone until the next evolution or you will make an unevenness in the finish that is very hard to card out.
Sun light is what will reveal how evenly the finish has been applied and will show any differences in depth and texture.
I use the same technique with Laurel Mtn.Forge but with no boiling. Instead the steel is carded by rubbing a course hot water wetted piece of burlap or canvas cloth. I also use the soft wire wheels and hand brushes purchased from Brownells with .003 thick wire bristles.
Niedner method is very sensitive to any oils so the work must be handled with washed linen gloves where as the Laurel Mtn. Forge work can be handled with bare hands with no ill effect.
Niedner also polished the work out to about 400 I believe but I much prefer the garnet sand and glass bead finish as it allows deeper penetration and looks about the same as the polished finish that is made porous by the solution pitting of the surface.
I use a bore protect-ant called "Hold" purchased from Brownell's which is made to apply to polished steel to prevent oxidation until the solution is applied. I coat the bore with this along with the tapered wood bore plugs/handles then drive in the plugs which seal the bore and protect it from any leakage. I have never found any need to change the wood plugs between boiling when so applied.
Niedner used shellac to seal their barrel bores but I have found it hard to remove so switched to Hold from Brownell's which is easily removed when finished and works just as well to protect the bore.
If one has the chemicals and wants to mix it them selves I can provide the technique and technical data as described in a book I have authored by Roy Dulapp but it is messy and some what dangerous so would recommend they purchase a bottled solution.
Remember Acid Into Water just Like you oughta, not the other way around. And run the acid down a rod don't just pour it in.