If you really, really, REALLY want your paint (and primer too, but I have never tried it) to be flexible, ask for a tube of flex agent. It is the stuff they put in paint when it is going to be applied to a plastic bumper, or panel that can be expected to be, well, flexed. Just be forewarned, it is not cheap. It won't hurt anything to be applied on non-flexing areas, but is a little expensive to waste.
Meddle not in the affairs of dragons, for you are crunchy, and taste good with ketchup.