yes, clay is a must after washing and before you polish/wax your car ...

purpose of clay is to have a clean surface for polish/wax ...

if u straight wax without clay, the dirt will together mount inside the car paint ...

my fren's car never clay before, that day he clay, the clay turn to black immediately ... so u can think how dirty the car paint is even washed with shampoo already ...

ralf, the proper way to see a car need clay or not is put a plastic cover(from cigaretes pack example) and then tough the car paint, if u can feel 'bintik - bintik' means it need clay, else, u can feel it very smooth ... however, as long as u drive out, for sure ur car paint is polluted by dust or stuff that can't wash away by car shampoo 1 ... means clay is a must before any wax/polish