Help! Weird 2-Toned Finish on Aluminum

[buffadiddle]

Here's the deal.....
I'm getting this strange 2-toned finish on aluminum.
It starts to appear somewhere between 320 to 600, during both wet sanding or steel wool,
before I move to the buffing wheels.
see picture
It stays like that all the way through the three buffing wheels.

What is this?
What's causing it?
What can I do to correct it, and/or stop it from occurring?

Thanks!

Mike

[customandsound]

that is the layers in the metal it is a bad casting you may be able to blend them in some more

[buffadiddle]

I think the piece is made from round stock, approx. 7/16 inch diam. Is that still cast aluminum?

In any case, any tips on blending? Should I level sand until I remove the top layer?

Thanks again,

Mike