Plating gold on aluminum packages used for microcircuits was my first engineering assignment as a summer intern, then I returned later as a plating engineer before slipping down the slope into printed circuits, from which one never returns.  After surface prep (very important as mentioned by another), copper plating, then nickel, then gold.  Here are 2 external sources that are used by Keysight.
Cal Aurum http://www.electroplating.com/contactus.html
EPSI http://www.elecprec.com/contact.html