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Joints in exterior woodwork always seem to open up a year or two after the work is done. In the short term it's a minor annoyance requiring a quick touch up. But over the long term it can be a major problem. Open joints are usually caused by two things. Structural settling can shift the position of individual boards and trim, and ordinary seasonal changes in humidity can cause enough expansion and shrinkage across the grain of wider boards to break joints open. Once a joint opens, water penetrates the crack and soaks into the backs of boards which then swell in width. Backpriming When installing exterior woodwork, carpenters usually build the work up in layers. First, back boards