Will this interfere with the trunk seal or lid operation?

No. This diffuser mounts below the bumper line and does not contact the trunk assembly. The rear lid closes and seals normally without any bind or interference.

Does the diffuser sit flush with the OEM bumper, or does it protrude?

It sits flush and integrated under the stock bumper. Because it uses factory attachment points, there are no gaps or step-downs. The result is one unified rear end rather than a bolted-on appearance.

What prep work should I do before painting?

Sand the primer surface with fine-grit sandpaper to promote paint adhesion, then clean away all dust before applying your chosen color or clear coat. Standard fiberglass prep—no special process required.