There are several grades of Dibond brand ACM. One has an anodized aluminum finish that would presumably hold up better outdoors. I don't know any shops in our area that actually use Dibond. The low pricing of the competing brands is too attractive.
An automotive polyurethane clear would, again presumably, allow an aluminum finish to weather longer. A catalyzed polyurethane gets harder, and is more resistant to moisture and sunlight, than most any non-catalyzed clear.
One word about Chinese manufacturers. Generally, they make what they are asked to make. Their manufacturing capabilities are the equal of any in the world. But when a buyer tells them to make something cheaper, they will do that, too.
There are codes that no longer allow the use of some of these ACM products for fascias and building cladding. They are a fire hazard. Dibond has a version with a fire retardant in the plastic core. But it's not cheap.
It's just like anything else. What we get is what we pay for.
And a lower price is not always about cheaper materials.
The Chinese have an advantage in regard to their labor force. Workers have lower wages, work longer hours, and may even live in dorms at manufacturing plants. All this allows them to make a product with a lower price tag, as well as respond to market demands quicker, even though the quality of the product may be high. This is why iPhones are made in China.
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Regarding cre8tivedave's peeling letters, my first thought was, "Could that be a clear plastic covering that is now coming off?" But I quickly dismissed the idea. I think it would be very difficult to make a mistake like that. But I do remember a screen printer who once printed a butt load of real estate signs and a number of them still had the protective plastic covering them. The mistake was discovered later by the bewildered customer.
Brad in Kansas City