Hi Peter,
I must admit to not knowing about technical aspects of the way Ali is produced,
but, in my bike racing days, all the chassis,s of our race bikes were ali & even after crashing a bike ( fact of life when racing
) we always managed to make the chassis frames come up like new after being gravel rashed,
As the sheets of ali you have will be non structural on the car, I can,t see what harm it would do to get the paint off & buff the ali up, maybe the floor being the only part that needs to "have strength integrity",
Now you know I like a bargain, thats what i,d be doing to make use of the ali you have aquired
.
cheers
andy