Will this interfere with the trunk seal or undercarriage at speed?

No. The lip is designed to mount at factory suspension points and sits within the OEM bumper envelope. Verify ground clearance on your specific setup before driving, especially if running lowered suspension—air dam height can change with ride height.

Should I use adhesive or drill new mounting holes?

This piece uses factory mounting points, so no drilling or adhesive is required. Bolt it to the existing fastener locations on your bumper support. If factory points are damaged or missing, consult your vehicle's service manual for alternate hardware options.

How should I prep the FRP before painting?

Sand the surface with medium-grit sandpaper to help paint adhesion, then wipe down with a tack cloth to remove dust. Apply your chosen primer and topcoat per the paint manufacturer's instructions. FRP absorbs moisture, so ensure proper primer coverage to prevent adhesion issues down the road.