I have fabricated a set of custom fitted splash guards for my Truck using wood. The plugs are smooth and painted with Two stage urethane base/clear automotive paint. I plan the make two part silicone molds for each splash guard.
the thickness of each ranges from 1/8" at the edges to approx. 1.6", they are approx. 17" x 10".
I need to select a resin that has some flexibility so they can be fit snug to the body to take up minor gaps yet ridged and strong enough to survive use. They also need to be paintable using two stage or single stage automotive urethane paints.
The Flex series is recommended for anything that needs to be flexible, there's even a video using one as a mud flap to boot. I think the higher the number is, the stiffer it is - though I could be wrong on that. I've got some Flex 30 that I'm testing and it's fairly strong so far and all I did was some pulling on it to see if I could rip it and not only did it not rip but it also went right back to shape. I believe it's supposed to be paintable and dye-able. Hope that helps.