Thanks Pargyle, interesting points.
The wood was bone dry (probably kiln dried), radiata pine from across the Tasman (New Zealand), and was from the beekeeping shop here (they import them). It is not an oily timber. All the paint used was the best quality exterior grade paint applied during appropriate conditions. The blisters have only appeared on the hive during the winter here...there has been no hot weather and no sun on the hive, but the hive is on a hill (common here) and is exposed to very cold drainage air on that side. It seemed to me that moisture inside the hive produced by the bees, is condensing on the cold surface of the hive wall . I noticed beading of water on the surface of that interior wall of the hive when I looked in, and the timber looked very wet there too. There were no bees on that side of the hive at all, and it is not a strong colony. The blistering is only on the cold air side.
No I haven't picked at any of the blisters but will replace the box when the weather warms up and take the paint off that side and fix it up.