I`ll put this in the simplest terms; If you don`t have the ability to (1) pre-treat the hub caps properly prior to paint (2) buy an OEM 2K expoxy urethane wheel clear (3) and have access to bake temps above 425 F, then the next best thing is a low bake (325F) powdered clear coat system, done by a reputable powder coater.
Or........... you can take your chances with a rattle can clear from the auto parts store and hope it maintains some level of adhesion or does not yellow in a short period of time and fall off.