THURSDAY, MAY 29, 2014
First of Two Parts
Typically used for siding, columns, beams, joists and fencing, dimensional lumber is finished, planed and cut to a standardized width and depth specified in inches (such as 2×4 or 4x4).
Length and stud size are specified separately from the width and depth.
Soft woods such as cedar, pine, spruce and fir are commonly used as dimensional lumber and often for exterior siding and fencing. These can be delivered to the jobsite either unfinished or factory primed.
Factory-Primed Problems
Dimensional lumber factory-primed by the supplier can present problems.
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Dimensional lumber delivered to the jobsite is often factory-primed by the lumber supplier. |
The primers used by lumber companies are generally of inferior quality and not suitable for the expansion and contraction caused by the temperature fluctuations and moisture intrusion that are common to most exterior applications.
So, the architect/specifier should take care to specify the required primer in the wood section of the specification (typically, an oil/alkyd or latex primer such as those approved under MPI #5 or MPI #6). He or she should also ensure that the specified primer is compatible with the finishing system described in section 09900 of the spec.
Finally, the spec should require that the lumber supplier provide verification that the specified primer was used.
Moisture Matters
Exterior lumber must be protected on all six sides from water intrusion, which leads to swelling, cracking, or cupping. Cupping occurs when the wood is wetter on bottom than on top and the center sinks, leaving the edges higher.
Any of these conditions in the wood can cause the same issues with the coating.
Water intrusion can be prevented or reduced by specifying end sealing and back priming. Back priming involves priming the entire component, including the back and edges.
All exterior wood components must be back primed before installation, especially cedar siding applied below masonry or stucco wall surfaces. Water that contacts the masonry or stucco can pick up alkali salts that accelerate the extraction of soluble tannins, leading to yellow or brown stains.
It is not possible to guarantee longterm protection if the backs of wood surfaces cannot be primed.
To seal end areas often left unpainted (such as the contact point at the rail or siding), boards are best coated on all sides, edges, and ends before being attached to the fence or wall.
Problems with Sap or Pitch
Many paint failures on new wood construction can be attributed to the use of green or uncured wood, which releases sap, partially solidified resin, or water after being painted.
Pine and fir often contain saps or pitch trapped in knots and deep pockets. These can appear after painting and cause the paint film to lift, causing blistering, splitting, cracking and staining. On knots and areas where sap has been removed, knot sealers can be used to seal surfaces and minimize or prevent further sap release.
Friday: Dimensional lumber goes to school.
Editor's Note: This article was excerpted from Coating Specialist Training materials and PQA Inspector Dave Lick and is reprinted with permission from the MPI (Master Painters Institute). MPI content describes best practices for commercial, institutional, and light industrial painting.
Tagged categories: Coatings education; Education; Good Technical Practice; Master Painters Institute (MPI); Wood; Wood coatings
