Wood Panel Shrinkage and Bare-Wood Witness Lines

      A discussion of how to prevent or repair the tell-tale bare wood lines that may appear on the edge of stained door panels in dry winter conditions. April 29, 2011

About 6 months ago, I built and installed a large maple vanity stained dark. I just went to look at it after the client called, and there are small areas on the flat panel doors where the unfinished wood is showing. The homeowner told me it was about two weeks ago when it was pretty cold outside when it started. The wood is maple and the stain is dark. I am not sure if they had an additional heater going in the bathroom or not, but there is a heat register built into the toe kick on the vanity. What's weird is if the panel shrunk a little, I would think that there would be a consistent line where the unfinished wood is showing, but it seems to be random spots and mostly where the panel meets the stiles of the doors, not on the top or bottom where the rails are. Any suggestions on a fix? I can bring these doors back to my shop to do this.

Forum Responses
(Finishing Forum)
From contributor J:
The sides, where the panels meet the stiles, is exactly where one would expect to see evidence of shrinkage. If it's not a consistent line along each stile, that's just because the paint bled between panel and stile in some places, or the panel is sticking in one stile and pulling out of the other.

From the original questioner:
I forgot to mention that I used the space balls in the rails and stiles before I assembled the doors.

From contributor T:
Contributor J is right - the movement is cross-grain, so one would expect to see it on the stiles, rather than the rails. It's possible since it's been six months that the humidity was higher and the panels were carrying more moisture and swelled. Now that the client is running their furnace or heaters, the panels have shrunk. I've had it happen too and the last time, I mixed up a little dye and ran it through my small air brush and you couldn't tell it had shrunk. You could also use a small artist's brush and likely take care of it.

It's also possible that as the seasons change, the panel will expand again, then back again, etc. If I were going to fix it, now would be the time. For future reference, if the humidity is high here and I'm building doors and using stain, I blow the stain all the way around the inside of the panel with an airhose just to be safe. I also run a dehumidifier in the room I store my lumber in when it's humid here.

From the original questioner:
Sounds good. I will try that with the artist brush and stain, and spray another coat of finish? I also thought being in the master bath, the moisture from the shower, which I figure they use at least twice a day, would have kept them from shrinking. I am just wondering if I should wait a little longer to see if they shrink more.

From contributor D:
Not sure, but proper ventilation that gets used in bathrooms will truly help the wood. Really, without the ventilation, we can't guarantee our work to the fullest. Explain it if that's the source.

From contributor T:
Depends on how many coats you've got on them now, what your topcoat is, and what your dmt is. I've mixed lacquer and thinner to make a very thin coat and put it on with the same artist's brush and it wasn't negligible, so experiment a little. I've also masked off and used a spray can.

Everybody's conditions are different. I have vanities in bathrooms where there's no registers, some with wall heaters, some with no fans, and occasionally the problem will occur where you think it shouldn't. My opinion is that it goes back to the original moisture content of your wood and the humidity in the shop at the time the door was fabricated. I use a moisture tester to randomly test my incoming, I use a dehumidifier when it's humid, and I use a humidifier in the winter when it's cold. It's 5 here today and the humidifier is running full bore in the shop and in the house. (I have one on my furnace in the house, but in the shop it's only in the room where I store my lumber. 37% is a good number to store at.)

From contributor L:
Bring in a hygrometer and see what the humidity is. It should be around 40%. If it is below 27%, tell your clients that they need to add moisture to the air to raise the humidity. This has to be an average. Winter is notoriously drier than the other seasons and wood shrinks from moisture content of the wood and not temperature.

From contributor T:
Again, it depends on one's location and circumstances. I have hygrometers in a couple of locations and I can maintain approximately 35-36% humidity. While 40% is quite comfortable, above 36% and I'll have moisture running down my windows and doing some damage. Wood shrinks due to removal of moisture. That's a natural occurrence in the winter months and is not based on temperature, but is based on how much the furnace is running and drying everything out. Telling a client they need to get the moisture levels to 40% may not be realistic. I'd have water streaming down my windows and a real angry client if I made that recommendation.

From Professor Gene Wengert, forum technical advisor:
I have seen operations that finish the panel before it is installed, so that as it shrinks when the humidity drops in the wintertime, it will not show the white color. Generally, you should make items such as this at no higher than 7.0% MC (35% RH max). This will allow for a little drier atmosphere without major problems. Maple has swirly grain, so that is possibly why you do not see a consistent white line. Maple shrinks a bit unevenly. To address this issue, some folks use veneer and a composite core, which is more stable than solid wood.

It is a bad idea to humidify your shop above 35% RH, as that will postpone the problem until the customer gets the product. Do not count on having the customer have a good RH in their home or adjusting their RH to protect the wood. However, if the customer goes much below 25% RH, they are being abnormal from the expected (a home has plants, cooking, bathroom showers, etc. that add moisture, so we seldom see under 30% RH). There is no way you can make a product for an extremely dry climate (especially a concern when the drywall and painting people turn the heat up to dry things out in a hurry).

To protect yourself, it is prudent to measure the MC of the wood you use to avoid using wood that is too wet for the dry climates.

From the original questioner:
What would be the ideal MC in wood before starting a project?

From contributor T:
The wood in most homes reaches equilibrium around 8% moisture content; higher in the low, humid South and lower in the high, dry West. The wood in many homes will be subjected to changes in moisture content from 6-10%, so I build at about 8% to average and reduce the differential.

From Professor Gene Wengert, forum technical advisor:
In locations where the outside is under freezing temperatures, the air inside the home will be 30% RH or less. This is 6% MC in the wood. The average piece of wood in a home will run around 7% MC average, but it is the low MCs in the wintertime that cause the shrinkage and poor performance. So, we need to target the lower MCs. In the wintertime, the wood should be no wetter than 7.0% MC. A few wet pieces of lumber will end up in several different panels and create problems, so we are really concerned about these wetter pieces, even if the average is okay. However, appreciate that if the wood is under 6.0% MC, it machines with a little more tear out at times and requires a bit more skill when gluing. There are some locations where the humidity is lower (SW USA) and some more humid (coastal areas), but throughout most of the USA in the wintertime, 7.0% MC is best.

From contributor X:
This is pretty common, as you never know what the moisture content is in the lumber when you buy it or if you buy your doors out. It usually happens the first part of the winter as the humidity drops in the house from the heat being on. I just went back with an artist's brush and touched up around the perimeter of the panel. The good thing is once you take care of it the first time, you're done. I do take a little extra time now and make sure and push the stain in between the panel and the stile and rails and have not had any problems since. I live in the Fort Worth area so our humidity is usually low compared to some areas.

From contributor M:
Unless you are buying your cabinet doors, stain your panels before assembling your doors. If you have issues while staining the assembled door afterwards, use shader.

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