Ok I have my rear bumper sanded down but there are some stress marks in some places. If I go ahead and spray some primer on it will it cover it up? If I did spray it, would the primer need to have a flex additive? I remember reading a thread where it said flex additive was really only needed if you were painting it off the car. If I dont primer it should I just sand all those spots down to the urethane?

 

you need to sand those stress cracks ALL the way out, or else they will eventually come back in your paint. And you don't need flex in the primer. I actually painted my gfx and bumpers off the car without any flex at all and there doing fine. Just not if i bump something