Greetings,
I know what you are talking about... I had 2 customers complaining about wipers blade 'jumping' after coating the windshield; (complaining but in a good manner, I always tell to 'call me if you notice something different because I warranty my services).
2 out of 2, after many tests (and even (at the first case) re-polishing and coating re-application), we had to switch wipers (which were thrashed!), nothing related to coating...
But what happened different? The coating app. Wipers were the same from when they bought the car!
For the second one I went direct on the wipers, switched with mine for testing (was compatible) and no jumping... Ohhh life.
The wipers were still 'jumping' before (I'm 100% sure), but people only start noticing after you let their car's flawless... The 'small problems' starts to appear when you get rid of all the big stuff.
Imagine, I thorough clean the glass, clay it, polish it, use a cleansing polish then coat with a very slick substance.
Learned the lesson, first thing I test when start washing are the wipers. Today was no different, I've tested before starting and customers wipers were 'jumping' - but test was conducted in front of him to avoid any misunderstood later. I've advanced: maybe we'll need to switch those to a new one...
In your case, you also learned a lesson. You should have done some 'bag test' on different areas and showed him how harsh surface really was.
Another thing I do a lot is taking pictures of swirls and scratches when car arrive, 'as it is', so I can also avoid anyone saying I've done any damage. I remove swirls and scratches, not install them!
Back to your thread, overspray removal is a simple task, but can be a complicated task, if not almost impossible task in some cases.
For you to give a precise quote for this job (to do it thoroughly and well done), you have to test a spot in front of customer.
Waterless wash some area, and clay it. At this moment, try to emphasize the 'number of passes' needed to remove the overspray you will be dealing with, also be aware of any paint marring your claying may leave - that will need (generally) to be followed by polishing.
If followed by polishing, you'll need to add some protection after work is done. This demonstration will be more than enough to convince customer how much $$ costs a work like this.
Then you tell him/her: I'll have to do this to entire car to guarantee full removal of contaminants, and that may take XXX hours to be done. My hourly rate is YYY, so that's ZZZ $ dollars, ZZZ-5% for you that looks a nice person.
I was able to get rid of overspray using only a thorough wash with hyper wash (various foam pre-soaks), then 'scrubbing lightly' with drying towel.
Other time I was able to use a fine grade clay bar... On another situation, I had to use medium grade clay, which left marring that had to be polished out.
The happiest moment in my overspray removal was when I could use nanoskin medium mitt and the overspray was vanished in little passes...
The worst moment was when I needed to sand down sprayed areas to fully get rid of them, any clay and even compounding was able to get rid of it (strange as it looks, that happened!).
I recommend you to clay entire car always you can, majorly on those situations.
I also recommend you to avoid claying / 'nanoskying' (lol) difficult to polish areas, so you may have a hard time removing clay marring.
Overspray is not as trivial as it looks, like I said above, some goes easy, and some may be a real PITH (pain in the head) to remove.
I would like to talk a lot (yet talked!) because this is one of the subjects I like most, I've tried to give you a general idea of how I try to deal with it, so you can take advantage of something to add to your way of doing things.
Hope I've helped,
Kind Regards.
