It's a urethane kit, personally I would probably just send it back since it is still unprimed. However you can fix a sag like that by reinforcing the inside of the bumper with a piece of metal. My kaminari front had a sag in the mouth because I left it sitting in a funny position for so long, but I just took a strip of metal, bent it into a L-shape and attached it to the bumper. *problem solved*. If you put a big can like the tanabe on your car, my advice would be to put heat tape above the can so you don't have to deal with the effects of the heat on the paint and the bumper itself.