It <em class='bbc'>is[/i] possible to "overload" with coats of Z under certain rare circumstances. I have observed this in my somewhat controlled panel test. As discussed in the threads on "smeariness," you can get excess, unbonded Z that "smears around." This is not a big deal and will go away with a Z7 wash or with time.
Here are factors that will help build solid coats:
1. Use ZFX from day one
2. Use thin coats, only
3. Use Z6 between coats
4. Beyond drying time, allow time for coats to fully cure before laying down another set of coats. I allow a week, and I live in a warm, dry climate (Los Angeles).
In spite of the above, at a certain point, if the car is a garage queen, the car is going to need to be exposed to the elements, wind and sun, according to Sal, to create a surface that new coats will bond to. If you kept on trying to layer a garage queen (and my test panel is the ultimate garage queen - heck, its a "clean, air conditioned house" queen) at a certain point you have a Zaino surface that is so smooth and slick that it is difficult for new coats to adhere to the surface (not such a bad situation, though -- smoothness and slickness is one of the things we are aiming for). I have directly observed what Sal is talking about on my test panel. Using ZFX from day one lessens the need for exposure.
I don't think this is a real world problem. You only run into it if you apply repeditive coats frequently, and then usually only if the car is a garage queen or your coats were too thick. And, if you do run into the problem, a Z7 wash followed by a week or two of time will allow you to start laying again, particularly if the car is getting outside.