I understood the factory GP4 cars had fiberglass flares riveted on. Most race cars would use FG where possible, especially in a vulnerable place like a fender flare.
A good resource for GP4 cars is Patrick Hals website, he owns "Andy", "Candy" a couple GP4 replicas and the ADA Pantera.
http://www.detomasogr4.com/cars.html
Julian
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