Glue Line "Creeping" from Woodweb
The poster who started this discussion was having a problem with the glue reappearing on finished tabletops, and asked for help. - Editor
"We are seeing glue line creeping in some of our dining room tabletops. We can't figure out what is causing this? It's not always on every joint, sometimes only a few. They always appear after top coating. We find if we re-scuff it and relacquer it, they stay down, but it is so frustrating. Please help!" - Holly
The first response suggested looking to the moisture content of the wood, and its drying time, as the solution. - Editor
"This is a common problem and a common misconception. The problem is the moisture content of your stock and the amount of time you allow from glue-up to planing. What you're seeing is the wood shrinking around the cured glue line. You need to allow your wood to reach equilibrium moisture with its environment. Also, be sure to check the MC of your wood. It should be around 8-10 percent." - Jeff P.
"Using any type of water-based glue, you need to give the joint ample time to be able to rid the joint of the extra moisture that it has gained because of the glue. This means a wait time of about 3 days. Yeah, you heard me -- days. I've had this problem on door panels. So now what I do is glue up, plane to thickness after it's dry which is usually a few hours. Sand with my rough grit and then let them sit for a few days. Then sand with my final grit after that. If you can't wait that long, use a glue/epoxy that is waterless." - Leo G.
The original poster responded to the moisture content suggestion, and provided further details about the glue being used. - Editor
"I have heard about the moisture content issue, so I have been leaving panels clamped overnight wherever possible and a minimum of four hours to try to combat that. We are using Dural white glue but have seen the issue with others and the old yellow Dural as well. All of our lumber is from standard hardwood suppliers so I assume it is kiln-dried to proper moisture content, but maybe I should not assume. What is the proper amount of time lumber should sit in my shop for before milling and glue-up? Most of our lumber is coming from the West Coast by the ocean and we are in the province next door in Alberta so maybe this is causing the issue to be worse than normal? We usually let it sit for a few days wherever possible, but we are a production shop and can't always do that. Do I need to be leaving it sit for a week or more? How long after glue-up to planing?" - Holly
That led some participants in the discussion to discuss the impact of different types of glues -- and what "creep" really means. - Editor
"I have seen some white glue joints that remain plastic (maybe this word will not offend the 'creep' police) forever. You could wait 3 days or 3 months, and the glue will continue to move out of the joint after repeated sandings. It is because it is a soft drying glue. Stay away from white glue. Also, while I'm at it. White glue is different from yellow glue. It is not simply a different color by a different brand." - Adam S.
"You might want to invest in a moisture meter to check your wood when it comes in. Depending on how you store it, it could take a long time for it to reach equilibrium with your area. Regarding the difference between white and yellow glue; there are certainly differences between formulations of PVA glue. Some are softer than others. This is not, however, a function of the color. There are plenty of softer yellow glues out there and plenty of harder white glues. There are glues such as epoxy and polyurethane that don't contain moisture. They tend to be very expensive and more difficult to work with. If you allow your stock to reach the appropriate MC [moisture content] and allow it to rest after gluing before you machine, you will likely do away with your raised glue line issues." - Jeff P.
"There are a number of phenomena that get described as 'creep.' People seem to like arguing over which qualify for use of this term. Then the wood science folks enter into the discussion sporting a lab coat and PhD; however, their opinions and textbook knowledge are not always consistent with the way things really work. A lot of guys who talk about this don't actually build nice things in a shop so there is a lot of really dumb info out there. The bottom line is that 'creep' can be related to the plasticity of the adhesive, or moisture introduced into the wood. From experience, I have encountered and resolved problems related to both. This brings us to a couple solutions.
"As others have pointed out, it is wise to wait as long as possible after gluing up panels before final sanding. In my shop production tabletops or door panels might get several days. A complicated lamination such as a butcher block countertop may be allowed several weeks. Quality work and furniture get several weeks. Avoid using exterior PVA formulations like Titebond® II or III, unless you truly need the water resistance. For most interior woodwork, they are not necessary." - Frank H.
Other factors in this issue - including the amount of glue involved, the cut of the wood in relation to the grain, and heat -- were also suggested as having an impact. - Editor
"The amount of squeeze-out and what you do with it will also affect this. I wait about 10 minutes and cut off the squeeze-out with a chisel. If you get big squeeze-out, the glue inside the joint takes longer to cure out." - Rich
"I agree with the previous points made, but the cut of the wood can make a difference as well. That is the orientation of the rings at the glue line. With flatsawn lumber it's possible to have vertical grain next to horizontal grain, which are going to expand/contract at different rates. Might explain why some joints move more than others. "- Tim G.
"I agree with Jeff: the color of the adhesive is not a factor. You can also get creep from the heat applied subsequent to gluing the panels, even if you wait three days. (The failure to wait three days will give a sunken joint ... a depression at many joints.) This is a good practice, but I doubt that it is the cure for your problem. However, from your comments it sounds like you may have a raised joint; that is why you can 'scuff it and then it stays down.' This raised joint is caused by the heat softening the adhesive and then the pressure on the joint (all joints are under pressure) causing the soft adhesive to squeeze out. (Creep is usually referring to a long-term movement, so creep may not be the best descriptive term.) So, do you use heat at the time of finishing to cure the finish? Or perhaps you use a sander or polisher that creates heat. The gloss of the finish makes such bumps, that cannot be seen in the rough, obvious. The cure is to change to a white or yellow adhesive that is less sensitive to heat...when it cures, a chemical reaction occurs that is not reversible." - Dr. Gene Wengert