There are several possible processes, but the most suitable one for applying a protective coating is brush electroplating, also known as selective electroplating. It is a means for electroplating parts which are too large for processing tanks, in field installations, or where only a small area (e.g., bearing area of a crankshaft) is to be plated. Brush electroplating can generally be used to plate all types of substrates with all the types of plating as in normal electropolishing.
A brief description of the process: The part is made the cathode, and a portable anode (usually wrapped in fiberglass) is passed over the surface while plating solution (electrolyte) is supplied to the interface, sometimes being pumped through a hollow handle to a porous anode. Metal ions from the electrolyte are thus reduced to metallic atoms on the substrate, i.e., electroplating occurs. Of course, any necessary pretreatment (cleaning, acid activation, etc.) must be done. The plating solutions are much more concentrated than normally used, to compensate for the shorter time and ambient temperature.
This general procedure is also be used for brush anodizing (usually, of aluminum) and electropolishing (usually, of 300 series SS), the main differences being reverse polarity and that these electrolytes do not contain metal.
For a more detailed description, see the “Selective Electrofinishing” article at
The best book on the process is Electrochemical Metallizing: Principles and Practice by Marvin Rubinstein (1987).
Some of the major chemical and equipment suppliers are Brooktronics, LDC and Sifco, which you can access from the following site:
Their sites give more detailed information.