If it's only down to the primer then you can use touchup paint. Plenty
of sources for that (you can start at your local friendly Autozone or
Pep Boys, bring you car's color code, which will be shown on the label
on the driver door pylon). I was pleasantly surprised to find an *EXACT*
match (as in, you literally cannot tell the difference) for my wife's
gray Taurus at Wal-Mart for a couple of bucks a can.
If it's down to bare metal then you should sand and re-prime the area
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