The pre-wash is required to get the easy to remove dirt off - as much as 95% of the dirt on your car! You need to remove as much dirt as you can before you do a normal hand wash to prevent scratching occuring. This is done typically by using snow foam.

Equipment You Will Need
Pressure Washer
Foam Lance (suitable fitment for pressure washer)
Snow Foam Solution

1. Make a cup of tea to keep your hands warm.
2. Set-up your pressure washer as per usual and rinse your car to help loosen up the worst of the dirt.
3. Squirt some snow foam liquid in the foam lance bottle along with a small amount (a few inches) of warm water. The highest dilution ratio snow foam is Meguiars Detailer at 400:1 - worth spending a bit more for.
4. Attach the foam lance to your pressure washer.
5. Pull the trigger of the pressure washer, adjusting the nozzle for angle of spread, and the top knob for the amount of foam fluid dragged from the attached bottle.
6. Cover the whole car in foam and leave to soak-in for anything between 5-15 minutes, depending on product used, and how dirty your car is.
7. Rinse your car down, and refill your cup of tea.

Equipment I Use
Bosch Aquatak Go
PA Foam Lance
Meguiars Detailer Hyper Wash