Q After finishing, we see several short rows of dots running along the length of lumber, the longest is about 6 inches. The dots and rows are uniformly spaced. This is very uncommon, although we recently have seen more. Any idea of what this is?

A This is a defect created when sawing or re-sawing lumber. In the sawmill, the feed mechanism needs to transfer a lot of power. Some devices use small rubber tires, some use smooth metal rollers with a rubber sleeve, and some use a metal roller that has a multitude of metal protrusions that push hard enough to actually indent the lumber. The pattern is long rows running the length of the lumber with the protrusions arranged uniformly. This is what you have.

For some reason, when the protrusions push the lumber, the effect can easily penetrate 1/8 inch or slightly more. When the lumber with the pattern from the protrusion is lightly planed, the defect spots are not removed. The effect is that a gray colored spot is created; the color shows up after drying and is more easily seen after finishing. The thinner the green lumber, the more likely the defect will remain.

In my opinion, this is a defect for you, but is not a defect for the sawmill or when grading the lumber. Perhaps, you need to consider avoiding a sawmill that uses this type of feed roller. 

Maybe the sawmill, being unaware of the issue, can decrease the pressure on the roller without affecting their feeding problems. Here is a picture of one type of feed roller.  

Q We are drying western cedar and notice that wherever there was a sticker, we have a light colored streak, but between the stickers is the more normal reddish coloration. Stickers were very dry to avoid staining. We air dried the wood carefully in our Northwest climate. What happened? In addition we had some black walnut, unsteamed, and after drying, we had light marks where the stickers were located on the lumber. Same, but with red oak. Help?

A The key to color development or color changes when drying is: “Differences in drying rate at high moistures will result in differences in color after drying.” This is especially true for almost all species of lumber sawn from logs that have been in the yard for several months or longer at temperatures above freezing before sawing.

Let’s apply this axiom to your particular situation, and these comments here apply to most all species. You stacked the lumber with very dry stickers. So, the wood in direct contact with the stickers dried quite rapidly at cool temperatures. This results in little oxidation of the color components in wood, so the wood in these locations develops a light color, or we could say that the expected color development does not happen. On the other hand, the wood between the stickers dried slowly, which allowed plenty of time for oxidation, developing darker colors. In brief, we have two different drying rates when comparing the rate under the sticker with the rate between the stickers.

The solution, assuming we want the dark colors, is to use stickers at perhaps 12% MC initially. We can often achieve this higher MC with stickers stored in small packs outside with some air flow and exposure to outside humidity, but protected from wetting from rain.

Special note for oak: Very dry stickers can result in drying under the sticker that is so fast that surface checks and honeycomb can occur over and under the sticker. On maple, ash and white wood: For the whitest color we need to avoid slow drying by using very dry stickers and low relative humidities with good air flow in the kiln…usually this also means that we must avoid any air drying, as the oxidation risk is highest in the first few days after sawing in warmer weather. Hence, in warm weather, we target having the white woods be in the kiln and under correct humidity within hours after sawing.

Q The middle lamellar (the middle sheet of veneer in our three- and five-ply plywood), has frequent cracks or openings. The face and back veneers seem intact. How do we begin to diagnose the problem?

A The first step is to get a magnifying glass, at least 10x. Next, ask yourself if the crack was present and open when the plywood was laminated. If it was open, it would have adhesive inside the crack. 

It is also possible that the crack was there at the time of laminating, but high humidity caused the dry surface of the middle veneer to swell shut, so no adhesive, or very little, could get into this preexisting crack.

A key clue is that it is virtually impossible to create a new check in the middle ply once the plywood is assembled. In other words, all these cracks are preexisting before laminating the plywood. How do we know? First, unless the plywood is mishandled there is not enough stress created. Second, dry wood is twice as strong as wet, so we need a lot of stress. Third, as the grain in adjacent veneers is at 90 degrees, this creates stability in plywood compared to solid wood.

SPECIAL NOTE #1: I have seen cases where laminated product was mishandled. One example is when a stack of plywood is lifted without adequate support to keep it flat. You can actually hear the cracking. Another example is when plywood is used as a bridge and is run over by a lift with a heavy load. We are dealing with decorative plywood and not load-carrying material.

SPECIAL NOTE #2: This same approach can used to diagnose face cracks. Virtually all face checks are preexisting. The cracks are not created after laminating. Often, face cracks will have finish inside the crack rather than adhesive.

SPECIAL NOTE #3. As decorative material, this plywood is not designed to survive intact when exposed to wetting with water. As it is a dry product, low humidity will not create new checks.