There are new products out there called Kleer and Azek. They are white PVC and made for outside. All you do is heat them up, they soften, and then you can bend it, pretty much, in a circle if you needed to.
We sell it at the lumberyard that I work at, and that stuff is great. AND, it comes white. You can leave it up without painting it, or you can paint it. Your choice.
We stock the Kleer stuff and I have guys that come in all the time and get this for that particular purpose.
It is a tad pricey, but the money you save in labor and painting is worth it in the end.
Hope that helps.
Have a good one!!!!