I wfp them , usually do the 7.00 until 9.00, It is a lot quicker and the customer is usually pleased there frames get a good clean as well.
Never had any complaints off my shops and never lost one due to using wfp.
If it is an individual shop and quite simple then i will trad it.
As for water being walked into the shop, what happens when it rains, also you are only using about 5 litres max on a shop which soon clears up, when i finish at 9.00 the ones i started first have almost dried so no problem with wet glass.
As for traffic fumes, yes you do get the odd shop which does not come up great, usually due to bad frames but you get to know which ones and you just trad them.
If in doubt explain to the shop if there is any doubt you will revert back after 6 cleans, this then gives them 6 weeks to decide if they like it or not.