Troubleshooting Finger Joint Telegraphing

Examining why finger joints telegraph through a gypsum undercoat and polyurethane finish. May 11, 2011

We are a moulding company. We finger joint short length radiata pinewood using PVAC D2 and D4 glue. After that we rip and mould them, then apply a layer of 0.2 mm to 0.3 mm thickness gypsum (a mixture of Calcium Carbonate powder and glue) over the moulding using an extruder machine. Sufficient time was given for the gypsum to dry, Then the mouldings were sanded. We then sprayed the mouldings with PU sealer and PU lacquer. Our problem is after a period of several weeks, we can see and even feel finger joint marks at the places where the joints are. Obviously the surface has moved but what is the cause of the movement? Was it the wood that moved or the gypsum has moved, and why? We need suggestions on how to solve this problem.

Another question: is Urea Formaldehyde glue suitable for finger-jointing mouldings? Can wood be finger-jointed with UF glue using cold pressed method? Would using UF glue solve the FJ mark problem described in the first paragraph. What is the best way to do stress test on FJ marks on mouldings (example put in oven, etc?) I donít want to wait for several weeks to see the results.

Forum Responses
(Adhesive Forum)
From contributor G:
Finger joints move. There is nothing you can do about it. The best you could do is to make the molding and coat it with you .2mm product. Wait the two-three weeks and then sand it. That way some of the movement will occur and you can sand it out. Other than that, finger joints move, there's nothing you can do about it.

From contributor A:
Wood does not move at a constant rate from one piece to another. It is not a metal or plastic. This is why people dislike finger jointed products. The glue cannot overcome this natural expansion/contraction. Burying it under an undercoater doesn't really help as well.

From contributor R:
It's possible that the water-based glue is swelling the wood at the joints, and when it dries out totally, it shrinks and shows the finger-joint. Why don't you try a small sample run with epoxy and see if it does the same thing- run another batch at the same time with the your original glue, and compare the two over time. Try the urea formaldehyde glue on a third batch, at the same time, for further comparison. Use the same procedures and set-ups for all of the test batches.

From contributor E:
Gypsum loves moisture. It may be that it is pulling moisture out of joints. As suggested, try a different glue.

From contributor D:
You may have to do some investigating on whether or not you can make this profitable, but it is common in boat building to encapsulate the wood. Mouldings I would think would need a very mild treatment. People have different opinions on what to seal with. I have used a weak solution of clear epoxy and acetone, brushed on. It penetrates deep, and plasticizes the outer layer. It will likely expand when drenched this way, and dry with the same grain raising you now experience, but after sanding it will be as stable as you can get it.