Not to much science in this test, but it´s difficult since there are many variables.
For instance if the guy in front of you in the carwash is a muddy 4x4 with gravel etc. then the experience can be so much more damaging!
haha yes I saw this on fifth gear when it was broadcast some time back, agree not very scientific and clearly biased to give an unusual result, ie the carwash did less damage, still some valid points to be aware of such as dont wash your car with gravel!