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Material Movement with Recessed Flat Panel Doors 
Question
The customer wants them painted white and I want to eliminate any little black lines between the panel and the 2" stiles and rails of the door. If the tongue portions of the panels are glued, clamped and pinned to the stiles and rails so there's no black lines between the two, are these doors going to bust apart? I should note, most of the doors are 18" or less in width. Forum Responses
From contributor J: I would pre-paint the panels. Either quick mask the tongue part not the rails and stiles, (glue probably will hold on the primer anyway). Also make a fixture where you can just slide the panel under a lip that gives you the amount of exposed lip to slide in, spray rotate, and etc. It’s real quick and there’s no masking. From the original questioner: To contributor J: The slide in template is a great idea. However, I might not have made my question very clear. I want to build the doors by gluing, clamping and pinning the panels in place. I want to do all of this to help keep the expansion and contraction of the panels from busting the stile and rail joints. This way, I can prime the whole thing, caulk the joint between panel and frame, and finish coat. I did this on some doors years ago and never heard anything back about a problem. But the older we get, hopefully the wiser and more concerned we get with quality craftsmanship. Though typically a panel wouldn't be glued in place, hence the use of space balls, due to the size of the doors I think I could build the unit, prime, caulk, and paint, and be okay. Any further analysis of yours, or anyone else’s would be greatly appreciated. I really don't like to use MDF. My clientel gets really annoyed at the mention of anything that's not real wood.
From contributor B: I agree with Contributor M and would use MDF as well. Baltic Birch is typically less flat, less dimensionally consistent, has a softer face, and is grainier than MDF, so you'll have more time and expense in priming and smoothing the surface. If my client absolutely insisted on real wood (a rather nebulous term), then I would use a maple-faced plywood. That being said, I don't see the need for pinning and clamping the panel if you are going to glue it in place, and caulk the seam between the panel and the frame during finishing. Wood movement in this case is nearly a non-factor, since your panel is an engineered and stable panel. If the panel and the rails and stiles have a proper moisture content, the joint should not separate. No space balls are needed unless you have a solid wood panel.
From contributor J: To the original questioner: Thanks - the only other material I can think of is MDO plywood, also called signboard. I have not used it but it is used extensively by the sign industry since they paint and paper the signs. From the original questioner: To contributor B: I agree about the use of maple. I thought about that but even though there are only 55 doors in this job, overall there's a significant difference when calculating the cost. Once we've primed, and sprayed 3 to 4 finish coats, sanding between each, all of which we would do anyway, they really look superb. The MDF may allow the omittance of some of the finish coats, but it's still MDF. As you can see we've already made some. We did four different styles of the interior panel for the customer to choose from. But my most major concern is the expansion of the wood and you seemed to confirm my thoughts on that. Being engineered, I didn't think there would be a problem with the expansion or contraction of the panels, but I thought it wise to seek counsel. The engineering of the plywood is also why I thought the 9 ply birch would be better. Does this scenario sound appropriate to you Contributor J?
From contributor A: We've always used the 1/2" cutoffs from cabinet backs for panels in paint grade flat panel doors. Why would a stable panel like plywood break a door if it's glued? Baltic birch is a waste of money, and like others have commented, it’s usually warped. Make sure you have sloppy fitting panels to allow for plenty of glue room and use high solids glue like Titebond III or moulding glue. From contributor J: To the original questioner: Sounds like a plan to me. I wonder if glue and pins aren’t overkill. Being plywood it isn’t going anywhere for a long time, I think one or the other is fine. Also the moulding glue Contributor A mentioned is awesome stuff. From contributor P: Would you be able to go to a 7/8" door and use a 1/2" groover on your cutters? You’ll not have to worry about movement within the joints and you can pre-finish your panels. I’m sure you’ll save time and peace of mind quality wise. From contributor F: Personally, I think a maker can make a flat panel door and machine the parts accurately enough to just assemble the door in the normal fashion and also use space balls in the panel groove to prevent panel rattle. I think you are trying to reinvent an already well invented wheel with this gluing the panel tongue to the styles and rails approach. For what it is worth, if you glue solid wood around a piece of stable material such as plywood, you are basically edgebanding and the solid wood is free to expand outward from the stable panel and would cause no problem.
From contributor T: I agree with what Contributor F said. I also use backer rod instead of space balls though.
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