I've only been working for about a month at my new company, but we do a lot of electroforming so I may be able to help you. We electroform with electrolytic nickel and with electroforming, you actually want the adhesion to be weak, otherwise you will not be able to remove the electroform from the mandrel. To do so, we passivate the nickel mandrel, which is essentially creating a thin "film" of oxidized nickel on the surface.
So, to me, it sounds like you'll want to do the opposite. From my (limited) knowledge, I would suggest a 5% acetic acid solution. Dip the part in the solution for a minute or so and then rinse with DI water. Because we are just trying to clean it, you may have to leave it in longer for your purposes, I'm not really sure. If this doesn't work, I would suggest increasing the concentration or the time in the solution.
Hope this helps!