Caution: Some of the methods used
involve caustic or poisonous liquids and fumes that may permeate an entire
house. Plastic gloves and adequate ventilation are mandatory. The work
area and you should be well protected from possible damage or contact.
Too often manufacturers leave many small metal parts on models with unfinished surfaces including moving parts, frames, wheels and trucks. In a large percentage of cases, paint in unsatisfactory. Manufacturers that provide coated finishes, usually send these out to platers and finishers. But room temperature blackening, greying or browning (rust) can be done chemically at the bench. Some methods are very toxic, while others use strong alkalis or acids. However selenium dioxide solutions work very well and safely on copper,brass, steel, nickel and sometimes zamac.
These oxidizing processes are advisable on moving parts, frames and trucks to avoid paint fouling any movement. Fine detail on lost wax castings will be sharper and selective buffing can highlight specific areas. In general these finishes are more durable than paint on metal. Since most oxides are relatively poor electrical conductors, they should not be applied to contact or pickup surfaces. Tires and other parts on loco drivers may be treated, but treads and flanges should not be. This produces an aditional visual effect of normal wear. Usually finishes can be removed easily by wire-brushing , scraping or fine filing and weak acid solutions work well..
A-West offers Blacken-It, which does a fair job in producing an almost black finish. Several years back, RAC marketed Hobby Black No. 2; the best blackening solution tested. After a search, the manufacture, Birchwood-Casey, was found. Listed under antiquing, they offer solutions for a variety of finishes on most hobby metals. These are aimed more toward commercial applications; however in their brush-on or dipping gun finishing line, several of the blue and antique finishes are available in 3 ounce bottles at your local gun shop. These react in 30 seconds to a few minutes.
PERMA BLUE for steel (except stainless) is a slower, less potent treatment, which yields a light greyish finish with the first application. Each subsequent application darkens the finish through gun metal until a grimy black is attained.
SUPER BLUE, for steel plus nickel and chrome alloys, yields a dark black on axles, Athearn weights, and wire.
Note: Adjust brightness and contrast for optimum viewing.
STEEL FINISH
Perma Blue on axles, no buffing.
Super Blue on Athearn weight, no buffing.
ALUMINUM BLACK for aluminum alloys including some zamacs, yields a dark gray to black on Roundhouse frames.
Note: Adjust brightness and contrast for optimum viewing.
ZAMAK ROUNDHOUSE UNDERFRAME
Center of sill and lower center panels buffed with bristle brush.
Remainder unbuffed.
BRASS BLACK for copper, brass or bronze yields a dark black on brass axles, lost wax castings and wire. Large areas are difficult to finish evenly.
Note: Adjust brightness and contrast for optimum viewing.
BRASS FINISH ON AXLES
No buffing.
Buffed with roto bristle brush.
PLUMB BROWN BARREL FINISH can provide various rusty brown finishes. Parts must be heated with a propane or butane torch then swabbed. Color is affected by temperature and solution.
Explaining better than I, on their site, there is a TECHNICAL ARTICLE covering the subject in detail.
For hobby use, not all the steps are required. However all oils, coatings and tarnish must be removed. In general color is controlled by time and solution strength. Dipping may be used for large smooth items, but complex surfaces are best swabbed or brushed to ensure reaching into small crevices. Grab irons, handrails, piping and other wire work can be pulled through wetted cotton balls. With care, cotton swabs can be used on already mounted parts without damaging paint or plastic. Since the reaction is time dependent, in some stubborn cases, repeating application may be required to achieve the degree of coloring or blackening desired. Due to variations in metal surface stresses and application swaths, colors on large flat surfaces may vary or appear streaky. Touchup can sometimes even coat or this can be used as a weathering technique.
For the very small quantities used in model work, pollution should be no problem.
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