Q. I manufacture solid wood, laminate table tops. It seems no matter what I have tried as far as drying the lumber before and after gluing, I still get the raised glue lines that can be seen in the finished tops. I use a finish sander when I use hand sanding where they do a final hand sand with orbital sanders to achieve an even, fine surface. I then send it to be stained and sealed. Once I have stained and sealed, I sand by hand before applying UV finish to tops. In most cases I still have the raised glue lines or even though you may not feel them you can still see the glue lines raised under finish. Thank you for your time and if you can offer me some advise I would greatly appreciate your experience on this matter.
A. Appreciate that the glossier the finish, the more slight imperfections will be obvious. Also, appreciate that it is going to take a bit of careful detective work to solve this problem.
The first step is to examine the joint under perhaps 10x magnification to determine if it is just the glue itself that is raised or it is both the adhesive and wood at the joint that is raised.
Case 1: Glue only raised.
With the woodworking adhesives we use today, all of them harden when they cure. So, if they appear to squeeze out later, it is because they have softened. They could be softened from some chemical in the finishing system, or they could soften (in a few cases with simple adhesives) when heat or moisture is added.
Where would the pressure, if they did soften, come from to force them outwards? In a table top or cabinet door, outside pressure could develop if the MC increases, especially at the surface. This increased MC would cause the surface wood fibers to expand and push the adhesive outward. Note that when the MC is reduced after this increase, the wood will shrink back to normal, but the adhesive will not. So, is your finishing system softening the adhesive and also adding moisture?
It is also possible that the adhesive itself is expanding, probably due to interaction with the finish or because of heat from sanding or a finishing oven.
In my experience, a wide (or thick) glue joint will have a greater problem; I cannot recall this problem associated with a thin glue line.
You can try to run a test where you finish only half of a panel and then see if the unfinished half has the same problem, although without a gloss finish, it may be hard to detect a small raised joint.
Another test for you to do is to use a piece of sandpaper on a block of wood instead of the orbital sander as your final sanding. Or try the orbital sander with fresh sandpaper and with only light pressure. I am thinking that the pressure from the orbital sander is heating the adhesive.
Case 2: Glue and wood raised
I have seen this problem when the glued panel dries a little bit after it is manufactured. What is happening is that the wood around the joint dries a little bit, but the rigidity of the glue joint prevents the wood right at the joint area from shrinking quite as much as the rest of the wood. Hence, the glue line appears to be raised a small amount.
Another possibility is that when you prepare the wood edges that are going to be glued, that the jointer, router, etc. is creating a lot of pressure (especially if the tool is dull or is removing a lot of material). This pressure creates what is termed “sub-surface damage.” In other words, the wood cells at the surface and slightly below are squished a bit. With moisture and with time, these squished cells recover and return to their original size.
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