hmm...how old is your car? when was it last clayed? your car might have no or very little contaminants in the first place...
I suggest that you feel the surface of your car. the soft99 brand that i mentioned in my previous post comes with a small plastic wherein you can use to "feel" your car's painted surface for contaminants.
if you feel nothing clinging on the paint, then it's got to be clean...
in claying, lubricate about 2 square feet at a time. rub the clay using a light pressure on a left-right-left overlapping motion about 4-6 passes, then follow it up with an up-down-up motion with 4-6 passes.
make sure you check you clay from time to time, knead it when necessary.
I my experience, during the first 2 passes of the clay, you will feel some sort of resistance caused by the contaminants. After that, the succeeding passes would be as smooth as glass...
