Claying is essential before attempting to polish a car or seal/wax it. There will be contaminants in the paintwork that won't be removed from the two bucket method.
You will notice after claying a car that it should feel silky smooth. The contaminants get picked up by the clay bar. Old bits of clay are really useful for removing brake dust from alloys
There's a good guide here:
http://www.detailingworld.co.uk/foru...ead.php?t=4829