A Sponge called Wood - Preventing Exterior Wood Rot

Wood is great. Its durable. Easy to work with. Has a very good reusability. There is also a downside. That is wood is a sponge that loves moisture.

A little moisture is okay. The fibers in the wood hold better and help with heat protection to a degree. This generally is in the 6%-10% range on a moisture meter. This range is where rot or mold typically do not form. A good carpenter is always thinking about how to mitigate water. Achieving this can be in spacing from sources of water or using flashing. But a low cost and less used approach is using effective sealers, such as oil primer.

Oil has a much smaller molecule makeup than water. With wood being a sponge, the oil will penetrate deeper and because of its molecule size, the bigger water molecules have a harder time establishing a presence. The main issue I come across, is if oil primer is used, it is not used in the places where water might come in contact. These areas generally are the cut ends or backside.

While all forms of protection need to be considered, this issue has come up on a lap siding replacement project with the ends being completely rotted along the chimney rake. On removal, it was apparent that the cuts were never sealed before install. Because it is difficult for any painter to apply a good primer to these areas, the wood becomes very susceptible to water.

Ultimately, by following the manufacturer specs, using quality materials, and keeping up maintenance schedules, it is possible to get the most out of the life your home’s exterior materials.

A can of oil primer is kept by the cut station. After each cut, oil primer is applied to the ends where unprotected bare wood is visible.

Depending on project, either Sherwin Williams Exterior Oil Primer or Zinnser Cover Stain Oil Primer is used. The Sherwin Williams is very strong and effective but requires a 24 hour dry time. Zinnsers oil primer is a good quality product with a one hour dry time.

By adding oil primer to the cut ends, you add a layer of defense for water to stay out. As seen here, the end is falling apart, and what is left of the cut has no primer or paint applied.