I hate to say it, but we have great success with tongue and groove PVC. It doesn't rot, paint sticks to it very well, and it works pretty much like wood without the grain. We have used just about every wood known to man and the only one available today that has given satisfactory service for long pieces where PVC is not suitable is mahogany. We are experimenting with a fly ash material that does not have the expansion and contraction issues of PVC. We'll see.
PVC is safer than wood according to the MSDS. I hate using it, but I hate throwing scarce resources - time and money - at inferior wood and paint of today.