I'm a bit confused at your question...
depending on the karat and colour, gold doesn't usually tarnish,
however the scope of reactions that are possible with heavily
chlorinated pools and spa/jacuzzi/whirlpools and chemical exposure
are a different matter.
Is it that you are seeking a sealant or a varnish similar to one that
may be used on brass. Marine varnishes work reasonably well,are
waterproof,fairly inexpensive though a pint may be the smallest
available quantity,and last reasonably long in daily wear- however
some customers may be more allergic to the varnish than the alloy if
the piece is fabricated with a low karat gold ( 10kt, or 14kt yellow,
or white golds; alloyed with nickel).
As for plating the piece- I don't comprehend what it is you are
trying to accomplish with a plating bath, what metal you are
considering using on your anode, or if you are trying to raise a
layer of pure gold on the unpolished contact side of the piece by a
type of electrostripping process. So if you offer a bit more
clarification and describe the unpolished gold you are using, i may
have some idea how best to respond to your inquiry.