How to clean pitting on center lines?

go over them once with mothers aluminum polish

then decide on the pits...
if they are minor try 600 grit wet sandpaper, if that doesnt get to them you may have to drop back to 400 grit (hopefully not)

then you progress.. 600 to 800 to 1000 to 1500 to 2000, removing the scratches from the previous grit. use an old kiddie pool with some water in it since it's easier to keep it wet and rinse it and keep going, and easy to clean up after with a hose lol.

then do mothers aluminum polish at the end.

after that wax them keep them waxed no matter what. if you dont keep them waxed you get pits

remember dont sand too hard or you make gouges that are harder to get out with the next grit. and DONT use power tools. it's too easy to create quick nicks if the spinning part bumps something