Hard to believe that the tailgate and rear bumper have no contmination. Those 2 areas especially the top of the rear bumper is always the grittiest. Use a fine clay like Sonus green so there is less chance of marring the paint. Try to do a few panels before kneading the clay again. I bet the clay surface will accumulate dirt for you see that there is contamination. It's just not alot. Good job keep it clean and out of the elements.