With timely repainting, an exterior wood surface can last for centuries.   The problem primarily occurs when the wood surface has been degraded due to weathering prior to the application of the primer coat.  The primer paint/wood interface is critical to ensuring the long-term performance of paint.  The paint failure will ultimately be manifested as blistering, cracking, and peeling of the paint.  It could result in damage to the wood and more difficult and costly refinishing.

Exterior paint will naturally degrade over time.  Until the degradation proceeds to the point where the primer paint begins to show, the paint surface can easily be repainted with a topcoat.

Outdoor weathering of unprotected wood can cause severe surface degradation.  Wood siding can be exposed for weeks or months before it is coated with paint, stain, or other finishes (coatings).  This weathering before coating can lead to chemical and physical changes to the wood surface that weaken the future coating/wood interface.  This interface is critical for adhesion of film forming finishes and the performance of penetrating stains.

This problem is minimized when using a wood-based composite siding (medium density fiberboard, Masonite board, etc.) that is sold with a factory primer.  However, this type of siding should also be painted quickly before the primer coat is degraded.  Also, the longer you wait to paint over the primer, the greater the chance that dirt will accumulate on the surface and weaken the paint to primer bond.  A clean surface is needed to facilitate long term performance for all finishes – interior and exterior.