Will anything get the marks out of my factory valve covers?

[Hendoo]

I have my 87 GN factory valve covers off and was wondering what the best trick is to get the marks off of them. Cleaned w/ parts cleaner but still have those dark marks within the outside of the cover itself. Will buffing w/ compound help??

 

[Boost231]

not sure what these marks look like but you can also sand blast them.

 

[just plain john]

I bead blasted mine and powdercoated them aluminum. Looks good, and oil or anything else just wipes off them without staining the porous cast aluminum.