One of the best parts of using a foam gun is to apply the area before you scrub. Unlike a bucket, you have to use a bush to apply the soap or cleaner. Soaking the area first gives the soap time to work and do its job. The soap encapsulates and lifts all the dirt and grime from the surface. Soaking first will help minimize swirling and scratching.
Reminder: Don't let any soaps or cleaners bake in the sun too long; this will create a film. Make sure you work sections at a time.