Raised Glue Joints on a Wood Countertop
Detailed analysis of why glue joints may show up as visible bumps or grooves on a wood surface in a location subject to moisture changes. April 24, 2009
Question
I recently completed a job for a customer which included large 1 1/2" thick cherry countertops. The countertops were glued-up using Titebond I adhesive. Several months later I noticed a slight raised and discolored bead the length of each glue joint. What causes this problem and how can it be avoided?
Forum Responses
(Adhesive Forum)
From contributor D:
I'm thinking it could be the Titebond I. Titebond III would have been my choice for something that has the potential for water exposure. What type of finish is on the piece?
From Jeff Pitcher, forum technical advisor:
It's not your choice of glue. Either grade will work fine for your application although the Titebond III will give you more water resistance. What you're seeing is a result of machining too soon after you did your glue up. During the first 24-48 hours after edgegluing the area around where the glue has been applied is slightly raised. If you machine the panel before the wood has lost this extra moisture you can end up with the result you're seeing. Another way that this can happen is if your wood has a somewhat high moisture content (in excess of 9%) during glue up and it is subsequently put into an environment with a low ambient humidity. The wood shrinks but the glue line remains the same thus creating the effect you described.
From Gene Wengert, forum technical advisor:
Another possibility, similar to what Jeff mentioned is that when the counter is wetted, the wood swells up. As this swelling is rapid, the amount of swelling is somewhat restricted. This develops compression set, if you want the technical term. The glue line does not swell as much as it is quite rigid. Then, the water evaporates and the wood shrinks more normally in thickness, but the glue line is rigid. The end result is what you are seeing. If the top is sanded when dry, you will see depressions at each joint when the wood is wetted, but the top will return to a fairly flat condition when it is dried again in the normal dry air in the house. Using a liquid waterproof coating approved for food contact will help this overall situation.
From the original questioner:
The finish is high-solids sealer, two coats and pre-cat. lacquer, two coats. Absolutely zero exposure to moisture. Typically I wait overnight before machining, however is your suggestion to wait beyond 48 hours before maching? These pieces did move from a less controlled environment to a highly controlled low humidity environment. Are there better suited adhesives that will minimize this problem? If I remove the countertops, strip, sand, and refinish is the problem likely to reoccur? Is there any salvage? I appreciate the advice.
From Gene Wengert, forum technical advisor:
I would encourage you to strip, sand, and refinish in place, as unless you can control the RH, the problem is likely to occur again.
From contributor S:
I'll be the odd guy and say it is the glue, and it won't really go away. Yellow glue never sets rigid, and is always somewhat plastic. I have observed many times that a glue joint raises on the glue line after glue up and subsequent surfacing. One day, a week or a month, it still raises. I first noticed it many years ago in some stave built oak columns. I sanded on the lathe until smooth and then went home. Then the next day they were raised. I sanded again, smooth, went to lunch, and they raised. I sanded again and I chased those glue lines for a month, finally doing a last sand and then on to finish. Upon final inspection in the completed bar, I saw them popping up again, but no one else noticed them. I now keep my eye out for this problem and go to a rigid set glue for glued to width panels and other situations where the glue may still squeeze out on down the road.
From Gene Wengert, forum technical advisor:
"Yellow glue" is a poor name to use, as it includes glues that are similar to the old Elmer's glue and was white in color. That is, color does not indicate (anymore) the type of glue, as yellow color can be added to what was a traditional white glue. As a result, yellow colored glues can remain quite soft and flexible. However, if they are truly soft, then the wood shrinkage and swelling will be so strong that they glue itself cannot resist and therefore there is will be no raised or sunken joints or a long time.
The problem of raised joints over a time period arises with any glue that is more rigid than the wood and so the joint does not change size at the same amount as the wood does, when the wood shrinks (or swells). The behavior noted by contributor S is typically what occurs with a fairly rigid adhesive and with wood that is drying out in a dry environment, such as a home. Note that the home RH drops over a month or two when going from September (50% RH sometimes) to November (30% RH). So, the raised problem shows up continually for several weeks or months until the interior RH stabilizes and the wood adjusts to this new RH condition.
From contributor P:
I have to agree with contributor S and Gene. I see this all the time, my work and others. The greater the humidity gradient, the more pronounced the raised glueline. Experience has shown me that the more dense and tight grained the wood, the greater the problem. I see it under paint grade work too. I know this may seem nuts to most, but this is one of the reasons why I have switched to West System epoxy for certain joints that I feel would benefit. I used to call it glueline creep, and with epoxy, I do not have the problem. Of course it makes a more visible line, so that it the trade off.
From Gene Wengert, forum technical advisor:
I write a monthly column in FDM magazine called "Ask the Wood Doctor." Here is a response from May 2002 from a reader and good friend of mine regarding this problem: This "bump" was a problem I discovered many years ago when sanding glued wood on an edge sander. I could not make the PVA glue line bump "invisible" to the touch. As soon as I turned off the sander and blocked the joint by hand, the bump disappeared and became smooth to the touch.
I looked at this phenomenon under the microscope and it was very clear that the glue actually bulged out of the joint. It was also clear that if the glue line were just a few mils thick (under 5 mils, I believe), this was not an issue; in other words, it seems to occur only with a thick glue line. My conclusion is that heat in sanding causes thermal expansion of the PVA. If the glue line is too narrow, then there is not enough glue to expand to the extent that it can be felt or seen. We find it most troublesome on flush rims glued to a panel. After finish, the bump may be apparent even from heat of finish ovens if the glue line is wide.
I have also reproduced this phenomenon when a dry, glued panel is exposed to high RH conditions, around 80% RH or higher. When the RH goes up, on wide joints the PVA actually seems to "grow" out of the joint. Again this only is an issue on wide joints, say above 5 mils and pretty common if the joint is 10 mils. This moisture effects seem a bit more puzzling because I am uncertain if high RH can actually make the PVA grow as much as I have observed. The growth or bulge is very significant. I theorize that the compression on the surface of the wood, coupled with the softening effects of high RH, squeezes the PVA out of the joint.
There are two solutions that have worked for me. The best solution is to make a tight-fitting joint; your column has certainly advocated that approach over the years. A second solution is to use urea or urea/PVA blend adhesive with perhaps 50% urea.
