Wood and Paint Adhesion

Whether the home is new construction or purchased from a previous owner, painting its exterior is the first step toward making it your home.  Likewise, if you’re looking to sell the house, an attractive exterior paint job goes a long way toward creating “curb appeal.”

The first step is choosing the correct the products (brushes, primers, and paint).  Second is to understand the relationship (adhesion quality) between paint and different species of wood.

How well exterior paints and stains adhere to wood is a direct result of the properties of the wood itself.  As example, dense wood shrinks and swells more than light wood, which stresses paint and solid-color stains.  This can cause the paint or solid-color stain to chip or peel prematurely.

Wood grain affects adhesion as well.  Edge-grained lumber holds paint better than flat-grained lumber because it is more dimensionally stable.  Also, edge-grain lumber has narrower bands of latewood, which is denser, smoother, and darker than early wood, and that, too, helps adhesion.

When painting, it’s good to know the difference between heartwood and sapwood.  Heartwood is the dark central column found in the center of mature trees.  It’s formed when individual tree cells die and are impregnated with extractives, pitch and oil.  This makes the heartwood of certain species (like redwood, cedar, and cypress) naturally resistant to decay.

Sapwood is the lighter shade wood that surrounds heartwood.  It is not decay resistant, but it normally won’t cause discoloration problems when finished with paints or solid-color stains.

Heartwood, however, can cause discoloration problems when it’s finished with paints or solid-color stains.  The extractives are water soluble, and when water is present in the wood, these extractives can dissolve and rise to the surface where they show through as reddish-brown discolorations.

Knots, or exposed end grain, absorb more finish than other parts of the lumber.  You can temper this by first applying a primer that blocks the extractives and then applying two topcoats.

When painting, start with the fascia (the body of the house) and then finish with the trim.