I will point out that if they are
drying their cars, that they likely get some splashing over the bay separator windows and so that is why we have a big parking lot. I sometimes tell customers this even if we are not full...so next time they'll get out regardless.
If a customer gets irritated, I will politely explain that this is a business and I rely on the money that goes into the machine to survive. They usually understand that.
If it all fails, I tell them to get lost...but that has only happened once in three years.
I also been toying with a message I could run on the PA, telling everyone in the bays that we are busy and to detail in the parking lot. That way you are not embarrassing someone directly by speaking to the problem and not the person.
I too see more people with their own chemicals. They'll do stupid stuff, like buy a bottle of degreaser and spray the whole car before washing. I then take the time to explain that we have similar products at a cheaper rate and that they are for clear coat paint and that using those chemicals is too hard on the paint. Sometimes I will show them the pH of the product with my meter and then how the appropriate stuff works just as well (like Bug Remover for bugs). This seems to work well as they know you know your stuff and are looking out for their cars.
BigLeo