I say with the Rustoleum paints that the flexibility is not an issue. I was asked the same question on another board, so I took my plastic paint tray, that I use to paint, and turned it inside-out. None of the paint cracked pealed or chipped. I have the flexible rubber front and rear bumpers too and have had absolutely no trouble with them from flexing during assembly. I would think that the Brightside paint may be just as good as it it made to go on fiberglass boats, with do flex as well, but I don't have any experience with it.