I am working on a project to figure out the best glue and material combination for our wide panels on entry doors. This includes, stain 2- sides, and stain/paint panels.
A little history:
Our panels are always 1.25″ thick made up of random width hardwood boards. We typically use Spanish Cedar, Mahogany, Red Oak, Ader, Maple and cherry.
We currently use Fuhr 106 H2O-based stain and 260 H2O-based lacquer. We use 3 coats of lacquer.
We started out using solid panels with titebond2 years ago. Anytime they were exposed to direct sunlight (especially if they were dark stains) they cracked all the way through (at a glue joint or sometimes elsewhere)or the glue joints showed as raised lines or split . This in turn creates a service call.
We build doors a bit differently than most. We mill details onto one side of the stiles and rails before glue-up. Then we use a matching stop on the other side. The panel is held in place with latex caulking, and the stops are caulked & nailed with 18g. brads. Or, sometimes, we use an applied raised moulding (with rabbet) for both sides. Both ways create a situation where, if the panel bows, it can create enough pressure to pop off the stops. This in turn creates a service call.
We then decided to sandwich a piece cheap 1/4″ doug fir 3 ply plywood in the center of our 1.25″ thick panel. So we have 1/2 of hardwood on each side to raise the panel with. This keeps any potential crack from going all the way through. We had bowing issues. Then we tried 1/4″ Medex and the bowing still continued.
We seem to have found our best results (as far as bowing is concerned) using a layer of high grade 1/4″ birch plywood in the center.
On a Stain/Paint panels, we now replace one side with Medex, still using the plywood in the middle. This paints easily and is more stable for the exterior side.
Gluing:
We typically use gorilla glue or a 2 part Urea-Form. 2-part glue from VacuClamp to edge glue solid 1/2″ boards. We have had recent failures with the latter that was supposedly a supplier issue. A glue joint opened up on a door that received a little sunlight. Only 1 of 5 joints failed. Then we use Titebond 2 for the sandwich gluing.
Interestingly, we usually only have the aformentioned underlined problems on DARK colored stains/woods and on WIDE panels at least 15″ or more. Let me also NOT forget that almost all of our failures receive some level of direct sunlight.
Here are my questions:
#1 What is the best glue to use in the edge gluing joints, and in between the sandwiches? We cannot afford these joints to open up regardless!
#2 I had a maple panel that was approx. 24″ wide x 15″ tall and ALL of the glue joints seperated after passing thru our paint curing oven at 95 degrees for 45 minutes 3 times. Is maple particularly bad to send thru the oven, or is this just a flat out glue failure?
#3 Is the plywood a good idea. Somewhere in my woodworking life I read that restricting the panels movement will automatically cause checking due to the panel wanting to move, however, we do not have this problem that often.
#4 Is it just stupid for us to try and sell stained doors in the sun?!
Awaiting your response,
Greg Madrigal
Replies
Hi Greg,
You've got a lot going on here, but I would say there is one root cause to all of your problems: you're not letting the solid wood panels move freely as they shrink and expand. As a result, the only option they have is to bow (expantion) or crack (shrinkage).
Q: "We started out using solid panels with titebond2 years ago. Anytime they were exposed to direct sunlight (especially if they were dark stains) they cracked all the way through"
A: The cracking is occuring because the panel is drying out in the sun and gettting stuck in the frame as it shrinks, thus causing the wood to crack. The panel should have a tongue that rides in a groove in the frame.
q: The panel is held in place with latex caulking, and the stops are caulked & nailed with 18g. brads
A: You shouldn't use anything to hold the panels in place other than a tongue and groove. The stops should not be attached to the panel, but rather the stiles and rails. Otherwise, they will prevent the panel from being able to move freely.
Q: We seem to have found our best results (as far as bowing is concerned) using a layer of high grade 1/4" birch plywood in the center.
A: This doesn't sound like a great solution. The hardwood veneers on either side want to shrink and expand. However, by gluing them to the plywood, your preventing that natural occurance from happening. The boards want to move so the only way they can go is to bow or crack.
Q: What glue?
A: Nothing is wrong with your glue choices. Wood movement is stronger than any glue or mechanical joint.
Q: Is it stupid...
A: It's not stupid, but you need to modify your construction process. We published this article a while backing on making interior doors. The same rules apply. You might also watch this video which shows a simple frame-and-panel technique that could be applied to your doors.
Good luck and write back if you have more questions.
Matt Berger
Fine Woodworking
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