Wheel well Protector

The only plastic panel I found inside my wheel wells was the plastic splash shield in the after section of the well. It, along with all metal surfaces inside the well, was coated with factory undercoating.

As far as any undercoating is concerned, if you start with clean, rust-free, metal, then prime and paint it, followed by application of good undercoating, you should be good to go. Proper finishing and application with modern materials should yield results equal to, or better than, what was done at the factory.