When the paint has been stripped from siding or any exterior wood, the ultraviolet rays of the sun begin to deteriorate the surface cells of the wood. The paint sticks to these surface cells well, but they may not stick to the layers of wood cells beneath them. The stripped surface should be touch-sanded within an hour or two of the time it is to be repainted to remove these sun-damaged cells, particularly if the siding is not repainted for a day or two.
When we were repainting a 1901 Victorian, we kept a palm sander on the scaffold to sand the exposed wood surface very lightly just before applying the primer. The new paint stuck well and has lasted. On the side that was painted by a contractor, who did not pay as much attention to the surface of the wood, the paint started peeling within two years.
Hank Spies, Champaign, IL