Drop in voltage required for electroplating?

[Jon Steensen]

I apologize if a similar topic has been answered somewhere else in this forum. I could not find the answer after a quick search, here or on google. If it is the case that I ask a question that has been answered before, please point me in the right direction.

I have a project where I consider electroplating a metal object with another metal lower in the reactivity series, but I am not completely sure I understand how it would work and which voltage would be required. E.g. plating a piece of copper wire with aluminium, using an electrolyte of an Aluminium salt of some kind in water solution. In the beginning this requires a voltage difference (which can be found from the reativity series), as a less noble metal is coated onto a more noble metal, that part is easy. I was wandering what will happen after the first thin layer of Aluminium has been applied? Will it then be like trying to electroplate aluminium onto aluminium, which in theory could be done without any voltage drop? (when the internal resistance in the solution is neglected. 
Even with a nonzero resistance in theory the smallest voltage drop would then be able to drive a very small current, which, given enough time, could do the plating. Albeit it will take a very long time with a near zero voltage). How thick has the plated layer have to be in order for the kathode to act as if it was made from Aluminium? Just thick enough to be waterproof? Alternatively, will the copper in the core always make it act as if it was completely made of copper?

Say there is a voltage drop when the part has been coated, then what will happen when more electrolyte is added, and the part of the cathode above the electrolyte level becomes exposed to it? Will the required voltage go up again?   Or will the Alumnium just be deposited on the part that is already coated, when if the voltage is reduced below the critical limit for alu-plating copper?

Edited May 4, 2018 by Jon Steensen
spelling correction

[Externet]

This has to be a good question.  Hope some experts will bring light.

 

Reminds me of when I was asked what surface would an adhesive be applied,  and others said plastic. Actually was epoxy paint on plastic, and that is the material the adhesive should be formulated instead.

And when choosing furniture cleaners,  picking the one for wood.  When the wood is actually sealed under layers of clear varnish.  The cleaner has to be for varnish, not for wood.

And selecting sandpaper for metal, for a car body... Actually the sandpaper will work  against old paint, fillers, not much on metal...