It worked well to make my roof spoiler

Now the next project is 2 small front lips

I thought I would make a 2 piece mold in fiberglass and that the final product would be 3-4 mm thick as the original bumper

Which polyurethane may be suitable? It needs to be more durable than the roof spoiler because it is located in a more exposed place

The Xencast P2 Fast Cast  becomes very hard, is there an alternative that is more flexible but can still be painted?