I posed this question to a bunch of people at D*con this year as someone asked me and I didn't have a satisfactory answer. Someone (and I wish I remembered who) said that they mist on a fine coat of spray adhesive like 3M Super 77 before they painted their flex foam armor with standard spray paints, and they've never had to repaint the parts since. Prior to that they were having to repaint the parts every time they wore them for an event.
I hope thats helpful or that someone has a chance to experiment and verify this method. Sounds logical to me to promote the bond with the spraypaints with a flexible base. If onlyI had any flexfoam parts around I could experiment with I'd give it a shot myself.