eibach, ground controll, and tanabe all powder coat there springs and have been for a long time..
the progressive springs i got for my bike are powdercoated.
my cuisine works at a powdercoater spraying commercial a/c cabinets, i hade him spray two 1/8'' x12''x12'' pice's of metal for my job, i can say there is a lot of flex in powdercoat.