I have had ideas to use actual car paint urethane to see if it is any tougher than the nail polish. There are different grades of nail polish but the high grades come in either lacquer or enamel.
Revlon makes possibly the best enamel and Smith and cult makes a really good lacquer that you actually use a hardener with.
Mopar colors would be nice too like EV2, FC7, EF8, or even X9 :D