urethane and fiberglass

Has anyone here tried molding fiberglass to uerathane? just curious cuz I have always been told it will crack. but I know some of the wide bodys for dsms. are fiberglass and ur supposed to mold them to the urethane bumpers. I was just curious cause I wanted to make somthing for my car by molding some fiberglass onto a urethane bumper. any help would be aperciated.

Thanks