Painting Dimensional Lumber
The Case of the Inside/Out Siding

BC Place

Excerpted from MPI’s Coating Specialist Training and the experience of Paint Quality Assurance inspector Dave Lick

Dimensional lumber is a term used for lumber that is finished/planed and cut to standardized width and depth specified in inches such as 2"×4", 2"×6", and 4"×4". The length of a board is specified separately from the width and depth; standard lengths of lumber range from 6 to 24 feet, as well as stud size: 92 5/8 inches.

Dimension lumber is used to make siding, columns, beams, joists, fencing, etc.

Softwoods such as cedar, pine, spruce and fir are commonly used as dimension lumber. These woods can be delivered to the job site either unfinished or factory primed. Softwoods such as cedar, pine, fir, and spruce are frequently used for exterior siding and fencing.

Problems with Factory-Primed Surfaces

Dimensional lumber delivered to the jobsite is often factory-primed by the lumber supplier — however, the primers used by lumber companies are generally of inferior quality and not suitable for the expansion and contraction that results from temperature fluctuations and/or moisture intrusion that’s 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 the products approved under MPI #5 or MPI #6 — while taking care to assure that the specified primer is compatible with the finishing system described in section 09900 of the spec.

The spec should also require that the lumber supplier provide certification verifying that the specified primer was used.

Problems with Moisture

Exterior lumber must be suitably protected on all six sides from water intrusion, which leads to swelling, cracking, or cupping (the condition where the edges of a board are higher and its center is lower because the wood is wetter on the bottom than on the top) in the wood, which then leads to blistering, peeling, and cracking in the coating.

Water intrusion can be prevented or reduced by specifying end sealing and back priming. “Back priming” refers to the practice of priming the entire wood component including the back and edges. All exterior wood components must be back primed prior to installation, especially cedar siding applied below masonry or stucco wall surfaces: water that contacts the masonry or stucco surface can pick up alkali salts that accelerate the extraction of soluble tannins from the wood, leading to yellow or brown stains. It is not possible to guarantee long term 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 attaching the board 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 that releases sap, partially solidified resin, or water after being painted. Pine and fir species especially often contain saps or pitch trapped in knots and deep pockets within the wood board. These can sometimes 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 from the surface, knot sealers can be used to seal surfaces and minimize or prevent further sap release.

The Case of the Inside-Out Siding

A 100+-year-old school building was to be renovated with costly new 6” fir lap siding custom-cut to match the old siding. Some noteworthy items about wood siding include:

  1. It is typically delivered to the jobsite factory-primed on all six sides to prevent the moisture problems described above.

  2. A board’s grooves and joints are fabricated so that there’s only one way to install them: each board has a definite “A” side (the ‘good’ side) that faces the exterior environment and will be seen, and a definite “B” interior side that faces the wall and won’t be seen after installation.

  3. A siding manufacturer’s processes are designed so that the “A” (good) side is handled carefully from fabrication to delivery so that the factory-applied primer remains intact and pristine — while the “B” side primer may have runs and drips as well as marks and dents acquired from tooling, contact with steel rollers and drying racks, or other stages of the handling and delivery process. This is not considered a problem because no one will ever see the “B” side after installation.

After all the new siding was installed on the school walls, the intrepid paint inspector arrived to oversee the finishing work and to his surprise, saw numerous flaws on the newly primed siding. Upon further inspection, he concluded that the siding manufacturer had made a critical error: the primer on the siding was, essentially, inside-out. The interior “B” side of the siding had a pristine and unmarked coat of primer, but the clearly visible exterior “A” side had the dents, marks, and runs typically found on the “B” side.

Now, what to do? The widespread marks and swirls in the primed surface were likely to telegraph through even multiple coats of intermediate and topcoat, which would lead to a highly unsatisfactory appearance for this newly-renovated building. The inspector ordered a mock-up on a section of siding to see if this was the case — and indeed it was: the uneven texture in the primer and surface flaws were still evident after application of the intermediate and topcoat. So all the siding had to be sanded and a full coat of new primer applied to cover the many bare wood spots present after the sanding work — which added up to unbudgeted costs and delays.

And if an oil-based primer was originally specified for the factory application, environmental restrictions would prevent this same product from being applied at the jobsite, so a waterbased primer compatible with the finishing system would need to be substituted.

How could this have been prevented?

Ideally, the general contractor would have inspected the siding when it was delivered, noted that the primer flaws were on the wrong side, and rejected the shipment. But the problem certainly should have been noted by the carpenter or GC after the first few boards were installed, and the installation work stopped. The remaining boards could have been sent back and replaced by the manufacturer, or the sanding and re-priming work could have been done on the ground or in a shop setting; either method would have been faster and more efficient than doing the work after the siding was installed.

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