Mill Glaze
Painting new wood it easy…right? Well, it does not have to be scraped and might not need cleaning. But there is a step that most people do not know, and it is an important one. When wood is cut the blades used to cut it creates Mill Glaze, which is a thin layer of glaze. The glaze is not a stable substrate to apply primer. Therefore in order to ensure that the paint does not fail, the glaze must be sanded off. In the picture below you can clearly see where I stopped sanding.
I did a job a few years ago on some clapboards. It still looks great. Except for one side. The boards on it are newer than the rest of the house and I suspect that the boards were not the pre-primed and whoever stained them did not remove the mill glaze.