How to Remove Pit Marks in my 68 Tail Panel

No, powdercoating your tailpanel will not cure your problem.
You will get back a tailpanel painted over with the pits showing as recessed areas.

The only cure to your problem is to get another panel without pits.

No matter what RustyRatRod tells you (and the multitudes of other over propagandized Members on here), powdercoating is paint and looks like paint, not polish aluminum. You will not be happy.
Do a post asking members about the big mistake they made powdercoating their Grilles.
And once it's powdercoated, you can't remove the crap, unless you grind it off.
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Suggest you clean the TailPanel up as much as poss and sell it CHEEP to somebody with a street car who can overlook the pits.