If you don't want wood that's painted or sealed to curl you need to paint/seal ALL the surfaces. Keeps different areas from absorbing or losing moisture. That's what causes the curl.I finished up the small deck for my above ground pool and I was really happy with how it turned out but I am having problem with the deck boards curling up. I went with regular pressure treated lumber and just finished it with thompsons water seal. I like the bare wood look but should I have stained it or painted it and that would have prevented the curling or warping? The framing of the deck is fine but should I have used a different wood?