Installing powder coated rims - sand down inside?
 

Just got my powder coated steelies back from the powder coating company and he suggested I take a hand sander to the back of the rims so when installing they aren't being installed directly onto the powder coat, but rather onto the rim itself. He said the powder coating can come off and create a loose gap between the two plates.

Anyone heard of this before?

Yes but you yourself sanding it, if the powdercoat cracks / flakes the painter can blame your sanding.

Honestly the painter could had taped up the contact points and saved you the grief.