I was working a thin (3mm) piece of aluminum plate with a rotating disc
on a hand drill. On the disc was a cutout of scotchbrite. *All of a
sudden* the aluminum went from having beautifully fine arcing scratch
marks to this hard marble orange peely crap. I assume that *something I
did was wrong* and that the work is now "work-hardened"?

If so, or not, how would I prevent such a thing? Is it a matter of use,
speed, heat, or what? And how can I remove it from the piece I have.

As always TIA.