The challenge is choosing a material that stays put in 100+mph rain, but doesn't pull the paint off when removing. As such, we chose a 3M material that has decent adhesion, but is classified as temporary, therefore less likely to pull paint off , etc.
If they are applied to a clean/dry factory finish, they will stay on during the races, but can be removed after the season without pulling paint. If they are put on over a rattle-can or a paint roller paint job, then they can/will remove some paint when removed, we found our the hard way on our car when we first decided to wrap it. Those hours I spent applying Rustoleum were for naught as that paint pulled right off when affixing decals. Also if the decals are hastily applied last minute in the rain to make the drop of the green flag, they tend not to do as well.
I am intrigued by the magnetic board suggestion, but I would hate to be black flagged if my number panel fell off. Maybe we could try some during the next race(s) along with the standard panels and see how they do.