Replacement Panel Coatings
There are two types of paint coatings applied to replacement panels.
The more usual one is a thin top coat designed to seal the surface from moisture and prevent corrosion during storage and shipment of the part. This would be a black coating which comes off readily with solvents and should be removed to allow an etch primer to bond to the metal surface.
Some panels are electrostatically coated, such as received from British Motor Heritage. This coating a dark brown color and is bonded very well to the base metal. These panels can be scuff sanded then primed.
Even with electrostatically coated panels, it would make sense to take external surfaces down to bare metal, then etch prime to make absolutely sure all the materials are compatible and the best paint bond achieved. The company responsible for the final painting will have input on this.
Some panels are shipped with only a thin oil coating. By the time these have been shipped across the ocean and sat on the warehouse shelves, these may develop a thin coating of rust, this can be removed by light sanding and chemical conversion before assembly.