My TR8 had the bumpers painted black and a clear coat applied by a previous owner, looked good but not original.
The finish was still good so I wet sanded them with 400 grit and sprayed them with rubber bumper finish.They now look brand new with a nice black satin finish, I bought the special paint at a body shop supply store. I took the bumper cover off of the rear to do the job but left the front one on and just masked it all off, all in all a nice afternoons work and worth the effort.
To get a nice look after washing apply Meguiars black trim dressing which is a white cream in a red plastic bottle.