I've recently looked up some info on PVA wood glue and found the following:
These gradings are part of the European standard BS EN 204 governing the classification of wood glues for non-structural applications.
There are 4 main types of durability grading, running from D1 to D4. The differences between these are listed below:
D1 = Interior areas, where the temperature only occasionally exceeds 50°C for a short time and the moisture content of the wood is 15% maximum
D2 = Interior areas, with occasional short term exposure to running or condensed water and/or to occasional high humidity, provided the moisture content of the wood does not exceed 18%
D3 = Interior areas, with frequent short-term exposure to running or condensed water and/or heavy exposure to high humidity. Exterior areas not exposed to weather
D4 = Interior areas with frequent long-term exposure to running or condensed water. Exterior areas exposed to weather
(BS EN 204 is a British/European standard.)
My question is: For painted exterior joinery (hardwood, with microporous water-based "breathable" paint), exposed to weather (windows and doors etc.) would D3 be acceptable, or must D4 classification be used?
I know it says that D4 must be used if exposed to weather, but presumably the paint protects the glue such that it is not directly exposed? On the other hand, the paint is breathable and so technically the glue joints ARE exposed to humidity.