What sort of contaminants?
I had the same trouble with a Camry several months ago. The owner hadnâ€â„¢t washed the car in 4 years and the front bumper was covered in bugs from highway travel. It was HORRIBLE. First I washed with Simple Green to loosen the shellac. I then tried a medium-abrasive body shop clay . . . the result was 60% better, but still far from perfect. I re-washed using straight vinegar, a soft toothbrush, and plastic razor blades. Better yet again, but not great. Finally, I dabbed areas with 3M Adhesive/Wax Remover and gently rubbed the debris free.