I have used the Krylon fusion on some of my exterior pieces, and it will crack/fade. If you don't mind doing it yearly, then it will be fine. Just clean the surface REALLY good, with rubbing alcohol, and let it dry. The spray light coats. I would put some clear on it, but that is up to you.
If I ever redo some of my plastic parts, then I will definitely use an adhesion promoter, primer, and exterior paint. But then again, if it is flexible, then Krylon Fusion would be best.
Ok, I have rambled enough. Just clean it real good, and spray light coats. It will be fine, lol.