On the hands and arms, the skin is thicker and the cream reacts only with the damaged areas. I would just apply a thin covering all over and the "good' skin would not be bothered. Should not be a problem, but if worried just discuss with your dermo. And if you have not visited their website, go to
http://www.efudex.com/treatment.jspf
Good luck, it will be better when you are done!