once a week
if you wash the car properly, you will end up with less swirls/sctraches then if you didnt wash it, all that dirt on the paint laying there will put some marring, esp if someone rus there hand over the paint with all the dirt there, as the dirt will do on its own in the wind or on the highway.
use a soft mit to wash, good car wash soap, two bucket system , after every section rinse the mit well, do the bumpers and skirts last(contains the most dirt buildup), and use either MF or wavvle weave drying cloths to dry the car.. this will ensure a safe, clean car
