I have a layer of Aluminum with a coating of Alumina. I want to remove
the alumina layer without affecting the aluminum layer below. what
chemical should I use for this pourpose>

I know of nothing that will dissolve alumina but not dissolve metallic
Al.  Alumina (Al2O3 or sapphire) is an extremely stable material.

I can't think of a selective etchant for that either, but Ill ask
around the lab. Can you electropolish or laser ablate the unwanted
layer?

Even if you remove it, it will quickly re oxidise unless it is
protected from the atmosphere.

Clutching at straws here. Aluminium solder flux breaks through the
oxide layer at soldering temperature -- but suspect it?ll go for the
underlying aly too. Any comments nanoalchemist?

Best

It is known that phosphoric acid reacts with alumina (not with
aluminum) and convert it into aluminumphosphate and gives off water as
byproduct. But the water produced again oxidizes the aluminum into
alumina. So if we have any other phosphate containing compound whose
byproduct does not oxidise the aluminum again. It might work!!! any
more ideas now.