Glue Joint Finish Problems
Joint movement associated with moisture in glue may be causing this reported finish defect at the joints. June 17, 2009
Question
After finishing with Chemcraft Plastofix L/S conversion varnish - 20 sheen, (both in the clear and black paint rub through/glaze and top coated with clear), the glue joints in the panels and the stile/rail joints are quite noticeable when seen in the light on an angle I am using cherry/clear and birch/black rub through-glaze on this project. This was not a problem when we used pre-cat products. There was no change in our prep procedure. Everything was sanded to 180 grit, stained (Tri-Clad), sealer coat, scuffed with 3M 2601 ultrafine sanding sponge and top coated (sprayed with pressure pot/hvlp gun). Does anything stand out as to where I went wrong?
Forum Responses
(Finishing Forum)
From contributor G:
It’s odd that you should think of this as a conversion varnish problem.
Glue Creep
From contributor M:
Maybe the joints are standing out more because of the sheen. I use Campbell products but I know some products will show mistakes more than others. Sounds like you need to prep a little better.
From the original questioner:
We were using Campbells pre-cat and this problem was not occurring. The panels feel and look perfectly smooth prior to finishing, but I can feel/see a tiny "valley" at each glue joint after the finish has cured, almost as if the wood is swelling slightly and the glue is not? Maybe it is a glue issue? The initial problem job had sat for around maybe 3-4 weeks prior to finishing, but I finished a pair of doors that were made and finished in the same day and still had the same results.
From contributor G:
Can you use Titebond II for glue?
From the original questioner:
Titebond - white glue. We were experiencing some black discoloration at the stile/rail joints after the door edges were profiled, so we tried the Titebond - white glue and the discoloration went away. We just started using the conversion varnish, but need to work out the bugs and get the correct procedure down. I like the product when it goes right, but when it doesn't it is trouble.
From contributor G:
When using Phenolic Resin glues I find that the glue line and wood surrounding it will shrink after sanding. This occurs because it is a water-based glue and it swells the wood fibers at the joint line. Then you sand it and then it shrinks back producing the line you are talking about. When doing doors, face frames and other glue ups I will usually glue it up, sand it down with a coarser paper and set it aside and do another aspect of the project. After a day or two I will come back and sand to its final grit(s). By this time the shrinkage has occurred and the water at the glue line has come out and equalized with the surroundings. Now the line shouldn't shrink. I know this isn't going to work for all projects, sometime you don't have the time to wait. But this technique should cure it.
From the original questioner:
We have some sample panels with different glues that we will spray up and check the results. The glue appears to be the main culprit. The pre cat used prior to us using the conversion varnish may have just been masking the problem.
