These watermarks will in most instances occur when your bench is new, and rich in colour. The marks will not weaken the bench in any way. The watermarks generally appear on the back of the bench, typically because the bench will be sited facing the sun – meaning the back does not get a chance to dry naturally. If your bench is backed against a wall, the effect will be exacerbated. To treat this, wash the whole back of the bench down and expose it to the sun to dry it naturally.