Will this wing stay stable at highway speed?

Yes. Single-piece FRP construction resists flex and vibration under load. Verify mounting bolts are torqued per your vehicle's service manual and that brackets are seated flush to the deck before driving.

Does the wing interfere with the trunk lid?

No interference with standard OEM trunk operation. If your vehicle has a spoiler delete or modified trunk latch, measure the clearance between the lid travel arc and wing base before final installation.

What prep work is needed before painting?

Sand the primer surface lightly with fine-grit sandpaper to accept your chosen base coat, then apply primer or base coat per your paint system's instructions. Standard automotive finishing steps apply.