Wondefull
What kind of plating do you use ? Electrolitic gold plating ?
Yep pickle the coin in acid to clean it
Then Attach scrap gold to one electrode and the coin to the other place in tank with plating solution and apply voltage. Around 1.2 volts works best for an even coat. 4 to 5 volts will flash plate but not very well.
Lots of nasty Chemicals involved including cyanide so not the kind of thing you want to dabble in. Will be attempting to make aqua regis, or nitro-hydrochloric acid next as it will hold gold in suspension and allow better plating.