Both over and under. Optimum themselves have said that you can apply clear bra on top with no problems. Since they say it's ok, I would do it incase the bra is ever removed, there won't be that part of the car without the coating. I believe the 3m is non-self healing, so no problem using the little extra OC to give it a little more protection. Some things will etch a clear bra and the OC will help protect it from that.