I have a nismo front bumper on the g it has a decent size crack in it can i use jb weld to repair the crack sand it down smooth and then wrap over it i do plan on wraping the whole car

 

JB Weld won't have the flexibility you need on a poly bumper, it will definitely crack.

I've used 3M TPO repair in the past and it works alright, I find it works best to prep the section behind the crack really well so you can put a big puddle on the back of it to help reinforce everything.

You definitely need to remove the bumper to repair it, prep the surfaces really well and you probably need to carve down into the crack a little to make a valley to fill with the TPO.