When you touch the paint it causes swirls. Period.

it is just a matter of how many and how bad.

I typically wash at home. On occasion I will use a self serve, BUT I bring my own bucket and wash/dry rags. I fill the bucket with soapy water, then wash.

All the usual stuff- control the dirt coming off, start high, rinse down, clean the wash mitt...

Conceptually it is simple: How to disrupt the dirt and get it off the paint as quickly as possible with as little interaction with the paint as possible.