Will this diffuser scrape on driveways or steep inclines?

The Backstop profile is designed with minimal extension to avoid contact on typical road surfaces. Verify ground clearance on your specific setup before final installation, especially on lowered vehicles or steep approach angles.

Does this provide functional downforce or is it cosmetic?

This is a styling piece. It enhances the visual presence of your rear end without generating downforce. Install it for aesthetic impact, not aerodynamic performance.

What prep work do I need before painting?

Sand the surface with appropriate grit to promote paint adhesion, then clean thoroughly to remove dust. Follow your paint manufacturer's guidelines for primer and topcoat application on FRP substrates.