When you're using a pressurewasher you start with the dirtiest piece(s) of the car/van or truck! This is always the bottom! If you strat at the top, the whole car is clean, when you reach the tyres, the dirt can splash back up on the already clean car and you have to re-rinse it! However, when you normally wash your car, you do start at the top and work your way down to end with the dirtiest parts!
