The lip is made of acrylonitrile butadiene styrene (more commonly known as ABS) plastic. ABS is not quite as flexible as polyurethane, but it's far more resilient than fiberglass or even carbon fiber. Plus, ABS is inexpensive to mold, so we've been able to further reduce the price on this spoiler.
Finish notes: The lips are made of black ABS or polyurethane. The finish is quite smooth, but there may be scratches or imperfections on the surface. If you're concerned about the appearance, we'd suggest painting with something like Rustoleum Paint for Plastic or Krylon Fusion for Plastics. If you do leave it bare, the polyurethane will eventually start to bleach from the sun.
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