The following was adapted from the Forest Products Laboratory’s Wood Handbook, Wood as an Engineering Material.
Decayed wood does not hold paint. Although this may be self-evident to some, it is one of the main tips researchers at the Forest Products Laboratory (FPL) have for contractors across the nation. Even in new construction, where new wood is expected to be free of decay, contractors can do several things to keep it that way.
If possible, paint all end grain surfaces with an oil-alkyd primer, such as the ends of siding and trim, brick molding, railings, balustrade, posts, beams, and edges of panel products.
When repainting, inspect wood for decay. Problematic areas include end grain of balustrade, brick molding, siding that butts against a roof, and the bottoms of posts on porches. Decay often occurs in the center of the wood, so the surface can appear sound; probe several areas with an ice pick to ensure the wood is sound. Replace boards having decay.
Decay and paint failure in a wood railing. Decayed wood does not hold paint.
Siding intersecting a sloping roof should have a gap (50mm) between the end grain of the siding and the roof shingles. Check for a finish on the end grain; if there is no finish, treat the end grain with a water-repellent preservative (WRP), prime, and top-coat. If there is already a coating on the end grain, keep it painted.
End grain of siding that butts directly against roof shingles (not a recommended construction practice) is not accessible for painting, however you can try to wick WRP into the end grain from a wet brush.
Insects seldom cause problems with finishes. However, when repainting a structure, it’s a good idea to inspect it for termite tunnels and carpenter ants. A termite tunnel is a sure sign of infestation.
Presence of carpenter ants may indicate decay in the structure. Carpenter ants do not eat wood, but they often tunnel out decayed areas to build their nests. Note that woodpecker holes often indicate insect infestation, as woodpeckers will destroy the wood to get to the insects beneath.
For more information, please see chapter 16 of FPL’s Wood Handbook, Wood as an Engineering Material.