Hi Mike, welcome to the Forum!
If all else fails, you may be able to refinish the surface by rubbing down with fine wet'n'dry paper used wet. As it's staining you want to remove rather than scratches the finer the better.
Start with as fine a grit as you can, then repeat with finer grits. Final rub down could be done with 1200 or finer steel wool.
It's possible then, if you need to, to polish up the plastic with cutting paste, T-Cut should do it.
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Cerwyn
Hobby collector of Replica model guns and Militaria.
also member of Living History Reenactment Groups.