Will this diffuser scrape on driveways or steep inclines?

This diffuser extends below the factory bumper line. Verify your vehicle's approach and departure angles before installation, especially if you run lowered suspension or frequent steep driveways. Concrete lips and speed bumps will require the same caution you'd apply to any rear bumper extension.

Is the downforce functional or purely cosmetic?

This is a functional diffuser designed to manage underbody airflow and reduce rear-end lift during highway driving. Real-world benefit depends on your speed and aerodynamic setup (splitter, wing, etc.). It's not a race-car-level component, but it works best as part of a complete aero package rather than standalone.

What prep work is required before painting?

Sand and prep the surface with appropriate grit to promote paint adhesion. Refer to your chosen paint manufacturer's surface prep guidelines for FRP. A clean, properly scuffed base ensures color and clear coat durability.