Sounds like treated 2x6 or 2x4. The problem is, they weren't intended for decoration. There is no cleaning them, you may get lucky with turning them over. If you replace some, make sure you pick them out yourself. If you look at the end rain you will see that they are put down so that if they do cup, the cup will be in the center of the board. If you turn them over they may cup up at the edges.