Aluminum needs a copper under-plate.
Nickel plating plates over copper super well, and that is why almost everyone who really cares about doing good work, uses a copper under-plate. I assure you any quality nickel plate job on machined, milled, extruded, or cast aluminum, needs a copper under-plate. Zamak or other cast alloy metals will also have a very inferior plating job, if it is not under-plated with copper, or brass. The same is true for steel, white cast, or grey cast steel, milled or machined high carbon, silicon steel, low carbon, etc. Gold plate also benefits from a cuperous underplating of copper or brass. Guitars get sweat on them for hours on end, and no underplating results in the plating bubbling off, pitting, flaking off, and generally turning into a huge mess. Japanese plating was notoriously bad for many years, but now China, Korea, and Indonesia are taking the lead in crummy plating. Yes, removing old plating is a massively huge PITA, but 98% of a new plating job in in preparing the metal. If it is not too obtrusive tiny bits of copper can be left behind, if they are in an inconspicuous locations, and the plating is thin, and feathers in well with the rest of the surface. It's best to go to bare metal, and you can use an etching gel to eat away at copper plating in tight places. Copper takes on nickel plating really really well, and that is why platers use it. If the nickel plate does not take well, it is right back to square one again.