Most dyes, such as Trans Tint, can only be mixed with alcohol or water, not mineral spirits. A water-based finish, such as Exterior 450, cannot be used in conjunction with mineral spirits, even with an extended dry time. Solvents do not play well with water-based finishes.

Unfortunately, there is no fix for the peeling finish. You will have to sand the project down, apply an exterior stain and recoat with Exterior 450. Also, advise your customer that no finish will withstand the sun indefinitely. All exterior finishes need recoating and maintenance over time.