Veneering - How much excess, and how do you cut it?

[Alan Lightstone]

Always wondered the best way to do this. I'm veneering a table top, and the sheets of veneer I have extend for a distance over the edges of the table top. When I put the glued/veneered top in the vacuum bag, how much excess should I leave on each side that will later need to be trimmed off? And how do you cut the veneer beforehand to avoid causing splits in it before gluing?

[Jamie Buxton]

I always make the substrate bigger than the eventual panel. The veneer extends pretty much to the edge of that oversize substrate. I trim to size after the veneer is applied.

I rough-cut veneer with a sharp knife and a straightedge. My sharp knife is a drywall knife -- cheap and real easy to sharpen. When I'm cross-cutting, I don't even cut all the way through the veneer. I score it, and snap it with the straightedge in place. Works every time. (You didn't ask, but I use a handplane to joint veneer edges before I tape them together into larger sheets.)

[Alan Lightstone]

Ooh. Great idea about making the substrate bigger. Unfortunately, wish I knew that about 1 hour ago. :(

I haven't had stellar success cutting with a drywall knife. Certainly not with cross-cutting.

Any issues with tear-out cutting the veneer/substrate sandwich to final size?

[Paul Symchych]

"Some" is my answer. I don't know about the vacuum bag but have never worried about the amount of excess overhang of veneer. It can be anything from 1/2" to much more.
To get it trimmed cleanly I suggest a veneer saw. Its a cheap little hand tool that will run around $15 or a bit more. It does a much better job than a box knife.

[Mike Henderson]

I make the veneer oversize to the substrate, and it's always nice to have the substrate oversize so you can trim it. The problem with making the substrate the exact size and then making the veneer significantly oversize is that the bag may pull the veneer down over the edge and break the veneer. And the break may extend into the show area of the panel.

Same problem with a caul (or upper platen). If you make the caul too big, the bag will pull the sides down and then you'll have an area inside the substrate where you don't get good pressure.

So I make the veneer about 1/8" bigger all around, and make the caul about the same, especially if the substrate cannot be made oversize.

I cut most of my veneer with a veneer saw and I trim the veneer back to the edge of the substrate with a block plane, most of the time. You could use a bigger plane under certain conditions.

Mike

[Alan Lightstone]

I've had the bag pull down the veneer and break it before. That was my concern regarding the oversized veneer.

Too late for me to keep the substrate a little bigger for this project. But you guys taught me a good technique for the future.

I didn't think of using the veneer saw to cut the veneer before gluing. For some reason I only thought of using it afterwards. I've typical used a router to trim the edges flush. I also worry about chipping with this. I usually cut the veneer close, then use the router for a more exact fit. But I've always felt I had too much veneer overhang in the first place.

BTW, can you glue one side, and then flip it and glue the other in the vacuum bag, or do you need to glue both at the same time?

[Jamie Buxton]

...BTW, can you glue one side, and then flip it and glue the other in the vacuum bag, or do you need to glue both at the same time?...

If the panel is thin -- like a quarter inch -- and you're using yellow glue, you're likely to see quite a bit of cupping if you veneer only one face. The water in the glue causes the cupping. If you wait several days, the cupping will diminish, but it generally doesn't entirely disappear. If you're using epoxy, which has no water in it, there's much less cupping. If you're veneering a thick panel -- like 3/4" thick -- you may not see much cupping with yellow glue and one side veneered.

[Jamie Buxton]

Ooh. Great idea about making the substrate bigger. Unfortunately, wish I knew that about 1 hour ago...

Substrate material is cheap. The work you're putting into it is expensive. Don't be afraid to throw away the small substrate.

[Mike Henderson]

You can glue one side at a time but I've had some problems with warping, depending on the thickness of the substrate - the thicker, the less problems. The problem is that most glue is water based. The water gets into the substrate and it expands and you have a convex warp on that side. Theoretically the panel should flatten out when it equalizes in moisture but sometimes it doesn't.

It's safer to glue both sides at the same time. But if you're working alone, and the panel is big, sometime you just don't have a choice.

Mike

[Jim Tobias]

I always like to cut the substrate slightly oversized and then lay it on top of the veneer and use it as a guide(while holding it down tight with other hand) for cutting the veneer. I like the veneer saw also for this process. As for glueing the veneer for both sides, I always glue them both before putting them in bag.

Jim

[Mike Henderson]

One additional comment. You probably know this. I only put glue on the substrate - not on the veneer. The reason is that some veneer, and especially in certain patterns, such as a radial match, expand when water hits it. So you only put the glue on the substrate, then lay the veneer on the substrate, tape it down, and then RUN to your vacuum bag and get it in press before the veneer makes like a potato chip.

That's one reason why I can't do both sides at the same time sometime.

Mike

[Alan Lightstone]

Big table top, so it will be a challenge to do both sides at once. I'll see how it goes.

The biggest problem I have is not having a vehicle large enough to transport the substrate, so I have to rent a pickup every time I pick up sheet goods. If not for that, I'd toss the substrate in a heartbeat and start from scratch.

Nice tips, guys. Keep them coming.
This will be on 3/4" and 1/2" plywood as the substrate.

I knew about this issues with warping and gluing one side of the substrate at a time, but with a 60" x 32" substrate of 3/4" plywood, it's heavy and unwieldy to flip. I was hoping I could separate the tasks.

Oh well, I'll have to sort it out.

[Mike Henderson]

If you use plywood as the substrate, make sure you use good plywood, such as Blatic birch. Lesser plywood often has significant overlaps and voids in it, and often has a prominent grain pattern in the outer ply. You'll find that these things can telegraph through your show veneer. If you just don't have any choice about the substrate, put an underlayment of veneer on the plywood first, before you put your show veneer (lay it cross grain to your show veneer). I often use mahogany because it's a fine grained wood, it's really not that expensive in veneer, and it comes in wide sheets (so less joints/less work).

Mike