it will a little but not that much. i mean to say it will stay and look straight but its not the sturdy as a fiberglass one would be. on the other side of the coin though, if you hit a curb, you wont break it like you would with a fiberglass one.
You should make sure that you bolt the center of the air dam to the frame, otherwise it will sag after time due to highway speed air pressure. Fibreglass is nice but can be trashed very easily as Zme stated.
At normal highway speeds, it's not a problem, but if you are trying High-Speed runs, especially in desert heat, do not use the urethane.
Ours folded under at about 150. Sanctioning bodies do not like air dams that plow dirt off the course!
But they're great for parking lot stops, curbs, and porcupines that you hit on the road doing 45. Mine's hit all those things and other than a little burr from folding under after hitting the proky, you can't tell it's 10 years old! Miracle stuff for the street.