Hi Mickwhitehill! I've never painted on Aluminium before, but there is a basic thing to consider to achieve the "metallic" effect you're trying to achieve. Most Acrylic paints (though not all) are opaque which would block any metal from showing through.
if you want to have a go at this, the best way would be to use only transparent colours (or as transparent as the system allows, and don't use any white, as this will certainly block the metallic show through.
Perhaps consider using Acrylic Inks, which would be a much more transparent system.
As far as de-greasing the aluminium, it probably is a good idea, as the acrylic may struggle for good adhesion on the metal surface.
Good luck and very best wishes,
PA & RSC Wigan area