Q: I recently applied a sheet of "peel and stick" veneer to a cabinet end panel. The cabinet looked fine from time of application until several weeks after installation. Now many bubbles appear under the veneer. How can I avoid this in the future?

 

A: I believe that the main problem is that the veneer is not well attached to the panel. Next time, very lightly sand the panel BEFORE you apply the veneer. Then remove the dust perhaps using a rag with a bit of alcohol on it (not Jack Daniels). Alcohol is non-grain-raising.

Why is this suggested and why does it work? Oftentimes, a panel is not active for gluing (there are a lot of spots for the adhesive to attach to the wood) due to the panel's age, dusty surface and so on. Sanding reactivates it, so you will get a very strong bond.

In rare cases, the panel substrate or the adhesive emits gas and this gas creates pressure that causes the bubbles. This is called off-gassing. I've heard of it, but never encountered it.

In extreme cases with lots of moisture changes (such as close to a sink or stove), which causes large differences in movement between the veneer and substrate, you can expect problems no matter what. Such problems will show up as having the bubbles or cracks all running the same direction (along the grain).

Sometimes you can repair bubbles by popping them with a pin or making a small knife cut in the center and then re-gluing them using a little heat to try and reactivate the adhesive.

 

Q: What's a good substitute for red alder?

 

A: It depends on what characteristics you're looking for. Certainly cherry looks similar, but is probably a bit more expensive. If you want a wood with similar grain but whiter in color, consider soft maple or birch.

Soft maple is a reasonable substitute in many ways (and will be cheaper), but the grain will be different. In fact, if you use a light finish, two adjacent pieces of soft maple can appear the same color and grain from one angle, but will have a high color contrast from another angle.

Here are some hardness values: Alder, 590 pounds; cherry, 950; soft maple (silver), 700; soft maple (red), 950 and white birch, 910.

 

Q: We received some lumber and checked it for stress and moisture. Some pieces were a bit high in moisture and we also noted some stress that we call casehardening in these wetter pieces. The supplier said it was fine when it left his plant and he thinks that moisture regain in transit caused the higher MCs and also the stress. Lumber drying seems to be a bit of black magic. Can you help us, please?

 

A: Drying lumber is indeed a specialized process. Each manufacturer and sometimes each customer have a special set of requirements for the dried wood. These requirements include target final average moisture content (MC), the spread of the average MC from piece to piece, the variation of MC within a piece (sometimes called the shell-core variation), the amount of transverse (across the grain) stress (also called casehardening) and the amount of longitudinal (lengthwise) stress. A lumber supplier has to guess oftentimes as to what is the exact level of quality that you want in these areas.

To assure that this final quality doesn't change between drying and manufacturing, the lumber must be stored at a particular relative humidity (RH) condition. Such storage is intended to hold the lumber at the proper MC and not to dry the lumber further or to relieve any stresses.

Equalized dry kiln

With respect to the particular situation that you asked about, it would be important that in the dry kiln the lumber is equalized (which means that all the pieces are brought to the same, or nearly the same, final moisture content). The kiln drying operation determines, by controlling the equalization process, how much variation of final MC exists. The smaller the variation desired, the longer the process of equalization that must be used. It is possible with proper kiln drying to achieve final MC variation that is so small that subsequent treatments or storage cannot minimize the variation further.

Once the lumber has been properly equalized according to the requirements, the lumber is then stress relieved, which is also called conditioning. The stresses can only be relieved by exposing the lumber to a rapid regain of moisture at the surface at high kiln temperatures. It is impossible to relieve any stress at room temperature. It's also impossible to relieve any stress without exposing the lumber to a high RH condition.

Special note: If the lumber is not properly equalized or if the lumber changes moisture after drying while in storage, the moisture gradient present will make the lumber appear to have stress. Such false stress will disappear if the lumber is stored at a constant RH condition for a long time. It's certainly more appropriate to equalize properly and then store the lumber at a controlled RH so that such storage is not necessary. In any case, the lumber with a gradient is stress free. I think your supplier is suggesting that this change in moisture is the cause of your problems . . . he may indeed be correct.

Store at proper RH

Once lumber is dried, if it is not held at the proper RH condition, it will change its MC in order to achieve equilibrium. For example, for 42 percent HR, the lumber will seek to achieve 8 percent MC, if it is not already at this level. Therefore, the lumber must be stored, after drying and during transit at the proper RH to avoid changes in MC.

Note that temperature is not a factor in the MC of lumber. Further, note that lumber that is wrapped in plastic, even if exposed to incorrect RH outside the plastic, will not change MC at all as no moisture can get into the outside of the wrapping. So, lumber shipped on a truck or even on a ship will arrive at the same moisture content that it was prior to shipping.

Here's the key: use a pin-type moisture meter and check the core MC. The core moisture will not change appreciably during transit. If the core is indeed dry, then your supplier is "off the hook." If the core MC is high, then the kiln was not proper equalized and subsequently stress relief could not be done perfectly.

 

Q: Is sapele part of the mahogany family or is it somehow considered a replacement for it? Thanks.

 

A: Sapele (pronounced sa-peel-ee) is a member of the Meliaceae family, as is African mahogany. Sapele's genus and species names are Entandrophragma cylindricum. It has grain and color that is similar to African mahogany, although some would say that African mahogany has a richer appearance. Sapele is just a bit lighter weight. African mahogany's genus and species are Khaya gradifoliola and Khaya senegalensis. So, sapele and African mahogany are not closely related.

True mahogany or Honduras mahogany is in the Juglandaceae family and is called Swietenia macrophylla. It tends to be a bit more reddish than African mahogany. Some plantation grown wood is less dense than African mahogany. Forest grown trees have been heavily harvested over the past four centuries, so most true mahogany lumber on today's market will be plantation grown and ecologically acceptable to use.

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