Rust & Paint Removal

[sgr60]

My hood is in pretty rough shape, so I wanted to see if I could get it down to bare metal and prime it. I had no problem taking the paint off the other body panels - using a DA, or abrasive pad or a flap wheel - I was able to go right through the paint and down to the steel no problem.

But, when I got to the hood, I don't think I'm getting there - it's really strange, so I'm wondering if anyone has had this -

Here's what the hood looks like after taking the grinder to it:



And a closer shot:



Obviously it's heavily pitted, but I'm wondering if maybe that black color is metal that has been treated with something? I can't go any further, so I don't think it's paint or primer, but I really don't know...

Thoughts?