My car is a show car. When I have a day to detail my car, I strip everything off the paint first and clean it w/ the claybar and all, so I start w/ a clean fresh slate each time. Then I do the swirl/scratch polish (z5) followed by the Z2. I usually do 2 coats of the z5, b/c it's all my paint needs, sometimes 1 or 3 just depending on how long its been and if I have swirl marks and all. But I can tell you from experience, that w/ every coat of z2 that goes on, the paint looks better and deeper. The z2 is the one that I always do atleast twice....most of the time 3 times. I don't do it just for the protection. My car is never in any type of element to need protection, except sun...and even that isn't too often. I do it for the appearance that it gives my car.
So to answer your question, yes, protection-wise, I think 1 coat is perfectly fine. But appearance wise, it changes w/ every coat I add.
