Caulking Wood Surfaces – Do not caulk bare wood surfaces. Caulk will not last as long if applied to sun-damaged wood. Wood that is exposed to direct sunlight for as little as two weeks will result in a weaker bond than normal. Caulk adheres best to primed or painted wood. The wood should at least be primed before applying the caulk and it will adhere even better if the wood surface has been painted first.
Just another expert idea from Guy Calor, the Caulk King and CaulkWarmer!