I want to rout a dado all the way across a finished plywood veneer with a thin veneer edge. Is this practical? Is it necessary to back up the thin veneer applied to the edge?
When I’m veneering edges, I typically wait until all the case joinery is done before applying the veneer. However, there are many times when this isn’t practical, such as when you have interior dividers and such.
If you’ve applied your edging with woodworking glue (white, yellow, etc.) you may be OK, but I would still take some precautions to prevent blowout, or chipping of the veneered edge. If you adhered the veneer with contact cement or a soft adhesive such as hot-melt glue (both gluing techniques that I don’t recommend), you are probably going to have some splintering, or even wholesale de-attachment.
I assume you’ll have a ‘left’ and a ‘right’ panel that needs dadoing. One panel will cut just fine because you can lead the veneered edge into the blade. The other edge will be exiting the blade, and that’s where blowout will be a concern. If you can lead the veneered edge into the blade, your problem is solved. However, many times this isn’t practical because it involves moving the rip fence for the cut, and accurately locating the left and right dadoes becomes a problem.
You can try backing up the cut with a sacrificial backup stick, but it can be tricky to clamp or otherwise temporarily attach the stick to the edge of the plywood. If you go this route, the stick must be firmly pressed against the edging to prevent splintering. Long-reach quick clamps might do the trick.
Another procedure is to apply some masking tape to the exit area of the dado, rubbing it hard onto the edge of the plywood, and then knifing the exact outlines of the dado through the tape and onto the edge, using a square and a sharp knife, such as a mat knife or razor knife. You’ll need to be very precise with your layout for this method to work.
One more ‘trick’ is to wet the edging with some water to soften the fibers, and then make the cut with one of the backup methods mentioned above. Wait a few minutes for the water to soften the fibers before making the cut, so the wood becomes more pliable and less prone to splintering.
Whenever possible, try using solid-wood edging to dress up your raw plywood edges. It can be anywhere from 1/8 in. to I in. thick (or more) and prevents a host of issues that a veneered edge poses. Solid wood edging is much more durable in the long run, and its thickness allows you to detail the sharp corners with roundovers, bevels or other decorative flourishes. When you do get some blowout on the edge of solid edging, you can simply handplane or sand the blemish away—something that’s impossible with veneer.
Thank you for your detailed sage advice. I should have explained that I am modifying an existing panel with a thin veneer edging applied with an unknown adhesive. Since I am removing some of the existing panel, I did an experiment with a router on the scrap piece. I found that I could cleanly cut the veneer on the entry side but as you forecasted, the veneer tore away on exit. After making this observation, I made several more successful dado cuts without blow outs by using only entry cuts on the veneer edges.
I hope the adhesive and veneer are uniform enough so that I will not have tearouts when I cut for show.
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