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