I second the comment to use the Miratec for this. I've used it in similar applications and have had success doing it and it still looks good a couple winters later. We did the trim around a bunch of windows in a brown color and it looks like new still.If you are going to paint, look at Miratec. It's available in 4/4 and 5/4 (3/4" and 1")
I like Azek but, it is salty if you desire a color other than white. The paint for the Azek is also kind of pricey.
Just another option for you.