Does this cap seal completely or do I need weatherstripping?

The cap mounts at the tailgate opening but seal quality depends on how tightly it sits once installed. Test-fit before committing to adhesive or additional gasket material — you may find factory mounting alone does the job.

Can I paint this before or after installation?

Either approach works. Painting before install keeps masking simpler; painting after means you match your bed finish without tape lines. If the cap arrives unpainted, prep with a light scuff and primer appropriate for the panel material before topcoat.

What hardware do I need to bolt this on?

Mounting hardware is not included with the cap. Bring the cap to your vehicle and verify the factory attachment points on your tailgate — you'll likely need fasteners that match those existing holes. Check your service manual for torque specs if you're using new bolts.