Look at the sensor basically the part is contained within a sensor holder, then pushed into the bumper. You will notice there is a tiny gap around the sensor - this is key.
Do not spray over this small gap - problems will ensue
Take care painting these items - the trick is to find/make a small cylinder of paper the same diameter as the one on the rear sensor push the paper onto the sensor x 4 then spray.
I've done afew over the years, but the black plastc looks fine.