Any harm in doing this? I'd hate to do it then find out something strange like it causes the diff to overheat or whatever. I doubt there's any harm in it, but better safe than sorry. Finally got around to spraying the wheel wells and I have 1/2 can left over. I figure the 3M would look better than rusty orange streaks.

 

I wouldn't do it. It'd be like adding a thin layer of insulation to it. If anything people try to do things to keep it cooler like adding a finned cover.