Will this diffuser scrape on driveways or steep inclines?

Rear diffusers sit low, so inspect your driveway angle before install. Test your approach at a steep angle before final fitment. If scraping occurs, you can sand or trim the contact point with basic hand tools to suit your specific setup.

Is this purely cosmetic or does it actually add downforce?

This diffuser is designed with functional geometry to manage rear airflow and reduce lift under acceleration. Results depend on speed and suspension setup, but it's engineered to work, not just look good.

How should I prep the FRP before painting?

Sand and prep the surface thoroughly before applying your chosen primer and topcoat. FRP accepts standard automotive finishes once properly sanded. Use a quality primer to lock in the gelcoat and ensure paint adhesion.