On mine the previous (factory) paint was just chalking continuously, but the paint itself was firmly adhered. I just washed it well using a brush, detergent and bleach, then applied "liquid sandpaper" to degloss and soften the surface. Then painted two coats of the Dicor.
2004 American Tradition; 2014 Buick LaCRosse
Homebase in the Ocala Nat'l Forest near Ocala, FL