Throwback Thursday : FPL Helps to Catch a Crimson Criminal

The following is an excerpt from Forest Products Laboratory 1910 – 2010 : Celebrating a Century of Accomplishments.

Wood science is not always about designing ingenious experiments or cutting-edge inventions. Over the years, researchers at the Forest Products Laboratory (FPL) have been called upon to use their expertise to help other agencies. Sometimes, this amounts to nothing more than uncovering decayed trees in a local city park, but in this case, it meant helping to convict a killer on the other side of the country.

In the “Case of the Red Paint Chip,” a murder had taken place in Georgia. The victim was found in the woods covered with debris, including chairs, rugs, cushions and two red painted posts. This area was used by hunters, who apparently brought the items and left them there.

The red posts, however, were thought to belong to the suspect, the husband of the victim. The victim’s son confirmed that red posts were stored in the house and that he had seen them in his father’s pickup truck.

In the victims house, under the stairs where the red posts had been stored, a red paint chip was found on a piece of yellow pine. This red paint was compared with the red paint on the posts, and a good match was noted in gross structure and chemical analysis. The forensic scientist in charge noted that there were a few fibers clinging to the back of the red paint chips.

Regis Miller, from the Center for Wood Anatomy Research at FPL, was asked to examine these fibers to determine whether they were conifer fibers from the yellow pine lumber, or fibers from the red painted posts, which were identified as from a species in the white oak group.

Sample and magnified view of the paint chip found on a pine board in the basement of the accused murderer’s home.

The ray cells did not have any cross-field pitting and no ray tracheids were evident, suggesting that they were hardwood ray cells and not rays from a conifer. These fibers were of two types: pointed thick-wall fibers and long, broad, thin-walled fibers. Fibers of two distinct types do not occur in the conifers, but they do occur in some hardwoods.

Miller concluded that the fibers were libriform fibers and vasicentric trachids that are typical of oak. Under high magnification and polarized light, the fibers from the red paint chip appeared to have large slit-like structures that initially were thought to be pits. Upon closer scrutiny, however, these slit-like structures were found to be the openings caused by decay fungi.

This decay pattern was found both in the fibers clinging to the red paint chip and the white oak posts. Not only were the decay patterns the same, they came from the same type of fungus. This case used both identification and comparative anatomy to show that the red paint chip found in the house did come from the red painted posts found at the scene of the crime.

Miller concluded that the, “analysis of the fibers attached to the paint chip indicated that the paint was from some hardwood posts that were found with the victim’s body and had been stored in the basement of the accused murderers home, and not from the pine 2×4. The paint was evidently chipped off when the posts were moved from the basement and used to help cover the victim’s body.”

Decayed Wood Does Not Hold Paint: Contractor Tips From FPL

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.

Scrape, Sand, or Torch? Removing Wood Finishes

When refinishing wood, it is sometimes necessary to remove the existing finish. Although seemingly a straight-forward process, tightly-bonded finishes or lead-containing paint can cause complications.

Scraping, sanding, wet or dry sandblasting, power washing, and using electrically heated pads, hot air guns and blowtorches are all methods to remove wood finishes—but do you know when to use which technique? Following are a few pointers.

Scraping is effective to remove loosely bonded paint or paint that has already partially peeled from small areas of the structure. If possible, sand weathered surfaces and feather edges of the paint still bonded to the wood. Do not sand if the old paint contains lead.

If paint has partially debonded on large areas of a structure, contractors usually remove the finish by power washing. This method works well for paint that is loosely bonded. If paint is tightly bonded, removal can be difficult without damaging the wood. The pressure needed to debond tightly bound paint from wood can easily cause deep erosion of the wood.

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Old finishes can be removed in a number of ways, but depending on the project, some may be more effective (and safer) than others.

If high pressure is necessary to remove paint, the paint probably does not need to be removed prior to refinishing.

If more aggressive mechanical methods are required, wet sandblasting can remove even tightly-bonded paint. Dry sandblasting is not suitable for removing paint from wood because it severely erodes wood along with the paint and it tends to glaze the surface. As with sanding, power washing and sandblasting are not suitable for paint containing lead.

Some power sanding devices are suitable for removing paint that contains lead; they have attachments for containing the dust. Equipment that has a series of blades similar to a power hand-planer is less likely to “gum up” with paint than equipment that merely sands the surface.

Remember, if the wood has fasteners, planers and sanders cannot be used unless the fasteners are countersunk.

Finally, paint can be softened using electrically heated pads, hot air guns, or blow torches, and then removed by scraping. This process is slow, but causes little damage to the wood. Blowtorches can be extremely hazardous, particularly if the painted wood is on a structure, as the flame can easily ignite flammable materials beneath the exterior surface.

Heated pads, hot air guns, and blowtorches are also not suitable for paint containing lead. Lead fumes are released as a lead-painted surface is heated past 700 degrees Fahrenheit.

For more information, refer to Chapter 16 of the Forest Product Laboratory’s (FPL) Wood Handbook.

Installing Exterior Wood-Based Siding: Tips for Peak Performance

Wood and water together can spell trouble. Lengthy exposure to moisture or extreme fluctuations in moisture levels can cause problems with wood, like checking, warping, paint failure, and even rot.

Proper building design and construction can help mitigate these problems. Well-known practices to prevent moisture accumulation within exterior siding include:

• Using dry materials during construction
• Providing adequate clearance to grade and drainage at grade
• Designing with adequate roof overhang
• Installing appropriate flashing materials
• Installing an interior vapor retarder in cold climates.

Proper installation will improve the performance of wood siding. Photo credit: National Park Service

Proper installation will improve the performance of wood siding. Photo credit: National Park Service

In a recently updated FinishLine fact sheet, another less-used but fairly simple technique is explained: ventilating horizontal lap siding by installing furring strips between the sheathing and siding. The fact sheet discusses materials and methods of installation in detail.

Although most of the article focuses on moisture, perhaps the most important factor impacting the service life of paint on exterior wood is also covered: exposure to sunlight.

Tests of wood painted, or at least primed, before any exposure to sunlight showed that repainting was not necessary for at least 30 years. Wood exposed to two weeks of sun prior to priming required repainting at 10 years. Applying a layer of primer immediately to wood siding will likely save time, effort, and expense years down the road.