Depending on which version of the helmet you're going to paint, I would suggest sanding, then repainting. I had the same issue, and actually, the subtle layers that you paint over add a bit of "interest" to the overall job when you're done. This is just a personal preference, but from the peeling i experienced, followed by the sanding, the paint looks great from afar and up close (almost like an extra weathering step). I realize that the original didn't have this, but it was a great side effect from a previous mistake!