I have obtained excellent results gold plating directly over nickel without any activation. BUT it has to be clean and free of oxides, and as polished as you desire the final gold to be. You won't be able to polish the gold much at all without going right through it.
For fear of wearing through nickel plate, use a metal polish as suggested by tbloss. Or you can use Flitz polish from Caswell, which will leave a rich glossy surface as well. Clean well because ANY metal polish will leave a thin film of wax as a sealant. This must be removed by solvent dipping and agitation thoroughly before attempting gold plate!
If you go through the nickel in the polishing process, you can re-nickel with a brush plate as a touch up. But beware. In this case you will HAVE to use an acid pickle to activate the old nickel in the area you are touching up.
When done with the gold, it helps to apply a thin coat of lacquer seal to protect it.
Ken
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