Some plated siderods seem to be more resistant than others to the effects of chemical etchants like gun blue or model railroad products like A-West Blacken It or Hobbyblack. In any case, if you're going to use these products on your models, it's best to disassemble things first. Many plated siderods seem to have a light coating of oil on them, which will prevent the necessary chemical reaction. With the rods removed, they can be easily degreased. Another thing that can interfere with the etching process is oxidation, and most products recommend that the parts to be treated be cleaned with fine steel wool first. This is something that should definitely not be done while the parts are still attached to the loco: a loose bit of steel wool can end up in the drive train, or be attracted to the magnets of the motor. The other reason to have the parts that you are blackening separate from the loco is to allow you to wash the parts to remove all active traces of the etchant before reassembly. If etchant is left on the parts, it can continue to "work", and may actually damage some parts - bearings and places where moving parts are in contact are particularily important areas where the etching process must be stopped.
This derelict locomotive, an old cast zinc model, was coloured using HobbyBlack, after first polishing the entire casting with steel wool. The resultant finish is extremely durable, and no sealant, such as Dulcote, was used. Any plastic parts are unaffected by the chemical, so you'll need to paint these details with a matching colour.
The manufacturers of these blackening products recommend that you not return the "spent" chemical to the original bottle, as it will contaminate the unused portion. I usually save the used chemical in another sealable container, as it often has enough "life" left in it to blacken other parts, but when the chemical process becomes really slow, it's almost reached the end of its useful life.
I did say almost: I transfer the chemical to yet another container, then, as metal "parts" become available, they're tossed into the container and left to sit, for days or sometimes even weeks. These "parts" could be broken metal couplers, damaged brass detail parts, or even brass trimmings from various scratchbuilding projects: if it's small and metal, and not useful for anything else, in it goes.

Eventually, the liquid in this container is almost depleted. When this happens, the contents of the bottle are dumped onto an old newspaper, then spread out to dry. The colours will range from black to rust, even green: perfect for a scrap pile somewhere or even a load for a gondola.
While I
have used HobbyBlack on siderods and valve gear, I generally prefer to use Floquil paints, applied either with a brush or an airbrush, as appropriate. Disassemble the loco, remove all electrical parts, then thoroughly wash the subassemblies with hot water and detergent, to remove all traces of oil and grease. Rinse well and allow to dry before painting. While the paint will rapidly dry to the touch, allow at least one week for it to fully harden before reassembly or operation. Be sure to lubricate all moving parts before operating the loco, too.
Wayne