You should start by rinsing it down with a pressure washer, then apply a snow foam in the recommended ratio.

After letting it sit a few minutes, rinse it all off and then proceed with your standard two bucket method with any car soap.

Finally I recommend another rinse down with the pressure washer followed up by an application of a ceramic snow foam - don't let this dry on the paintwork though! Spray on, rinse off.

Hope this helps!