Ford alum valve covers getting black lines in the metal when it starts to shine.

[RogFE427]

I have been told that this is a defect in the casting is there any way this can be polished out or does it have to be ground out?
Can this be sanded out or polished to where you can't see it, any help would be greatly appreciated.

[baz]

Re: Ford alum valve covers getting black lines in the metal when it starts to shine.

a good start would be a picture so we could see what you are talking about.