Sounds to me like you should have discussed care and upkeep with them.
Don't assume that we didn't. In fact, that is exactly what I was recommending the OP do. Talk to them about it. Let the client know that you actually have to care for the gold leaf. As you probably already know having family members who are firefighters, a firehouse gets new bloods all the time. Apparently how to take care for the gold leaf lettering isn't on the top of their list when training new guys.
You will hear no debate from me on the the differences in appearance between the real deal and the imitation products. We still do it here too, but for similar reasons why we don't hand letter every semi tuck that comes in our doors, there are time where we would ask the client to consider all the possibilities when it comes to lettering their fire trucks too.
And as lackluster as the imitation products look next to the real thing, anything looks better than gold leaf that has been scrubbed down to the size.
I'm bias too. I personally prefer the real thing myself. It's a hell of a lot more enjoyable to do. But I can see where the imitation products might have their place too.
You feel strong about what is the lifeblood of your company, and I can respect that. BTW nice work. All I'm trying to do is share our personal experience on the subject.