I have called Chemetal at one point a few months ago because a client of mine took a job using a Chemetal Laminate for a complete apartment signage sign system (against my recommendation). Over 150 k worth of signs, they insisted interior use only for all their products. This is 1 year after installation, aluminum deterioration with a clearcoat face and epoxy coating on back.
For the years you ( and I hear a few others including the sign company that did the failed job) have been doing it without failure, I have the same years seeing many failures. My job is lots of spec writing for large sign jobs, if it has no exterior usage and does not have a certain rating or quality specification, it's going to be a risky product to use for exterior use.
Read the fine print, proceed knowing the possibilities...