You can just scuff prime and paint it like any other surface on your car. The only downside of that will be if you hit something (cement stoppers in a parking lot or a curb) that your paint will get those spiderweb marks in it and begin to flake off. As for any other conditions (hot/cold) nothing happens. You can either add a flex agent to existing primers or get the primer with it allready mixed in. I'm guessing the second is what was mentioned just above me. I painted my mouldings and sideskirts silver by just scuffing/sanding, priming and painting. It's held up fine for like over a year now, so imo the flex agents are nothing more than a gimmick.
"Excuse me, is your refrigerator running? Because if it is, it probably runs like you... very homosexually."