You wouldn't need any flex agent, but yes, I usually do spray one coat of adhesion promoter before paint.
Another thing I like about bc/cc is that the color and gloss is totally up to the customer.... any color, any degree of sheen
well if yer going for OEM finish on the cladding, GB looks completely OEM. slight slight satin finish, but no matter how close u look, it doesn't look like a "top coat" like a BC/CC might.... it is "one" with the cladding.