Edge Joinery for Pre-Finished Back Panels
From contributor P:
I've been pocket screwing that sort of thing with good results.
From contributor D:
Edge joining pre-finished panels is tough since you can't sand the joint. I'd probably rout a 1/8" rabbet on one side of the joint so when I joined them, it would hide any minor imperfections without sanding. If they are equal sized panels, that is one thing you could try. You are going to see the joints no matter what, unless you can sand and refinish, so at least it will look like they are supposed to be there.
From contributor T:
I've had to do that a few times where I needed vertical grain for 8 feet or so wide and I've always just biscuited the joint, glued and clamped it. Make sure the biscuiter is accurate and dry fit it, have a good clean edge. If you select two panels with similar grain they'll blend pretty well and really not be noticeable to anyone but the skilled. Other options are to veneer it (outsource?) or to make it a frame and panel detail to break it up.
From contributor J:
I vote for frame and panel overlay and pin nail it. Biscuit the joint then cover as above. Will look great and save all the weeping and gnashing of teeth.
From contributor V:
In addition to the above methods I sometimes bolt panels together from behind with countertop draw bolts.
From contributor C:
Have you considered ordering plywood with the grain running the other way? No joints in an 8 foot run that way. I do that whenever I have an island that needs the back covered.
From contributor M:
You might try this - it works for me:
1. Clean up abutting edges - I have a simple flush trim jig made of 2 pieces of 1/4" MDF. Usually 1/32 or less is all that is needed to get a really clean edge.
2. Rout 3/16" - 1/4" groove with slot cutting bit in both edges. Protect surfaces!
3. Use either 1/4" MDF or mill out (saw and surface plane) some 3/16" maple or poplar for spline material. Spline should be 1/16" less than the overall slot width. Just break the edges with sandpaper to make it easier to insert, as the spline should be a very snug fit.
4. Dry fit - should be right on if you did everything right. Keep the router base really down flat when you run the slot cutter. Make sure the material is solidly supported from beneath (0 deflection).
5. I don't glue the spline in - just go dry and use glue on the 2 other edges on either side of the slot - you don't need a whole lot. This joint will never be stressed, so strength is not an issue. Have a wet rag handy for squeeze out. Set one panel in place as normal, glue up and press fit/slide the other panel in place. Usually hand pressure is enough to draw it up tight. If not there's always a way. The point is a spline is the best way to keep the 2 pieces aligned the whole length of the joint. You've got to dial in your spline to slot for a very snug dry fit, but after that it's not much more work than running some biscuits - way too sloppy for what you want to achieve. I have several lengths of spline (I prefer 3/16" so I can dial it in with my surface planer) material on the truck so it's always there when I need it. Like everybody else said, if you can't sand the joint once it's together, either disguise it or try the spline joint and see if it meets your requirements?
From contributor O:
Join them the best you can and trim every 12", 15", 18" or whatever it takes to be evenly spaced so it looks like it is supposed to be that way. No one will know the difference.
From contributor W:
I have used the same method as contributor M many times with great results. Just make sure the spline fits snug but not too snug or the joint will bubble.
From contributor E:
Collins Tool Co. sells a router bit that is supposed to help with this. It takes a little out of the plies so you can pull the veneer closer together with clamps, pocket screws, etc. I haven't used one but it looks like it might help you out.
From contributor G:
Hafele Furniture and Cabinetry Catalog. Connectors and Shelf Supports, pgs 303-483.
From contributor B:
Try having the shop tongue and groove on a shaper, 1/4 x 1/4 x 1/4 (with a power feeder no problem). Reference face down and you just have to clamp them up. We have done this with 3 sheets to make a 10 foot x 8 foot walnut panel.
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