Veneering a Half Cylinder
Detailed comments on the choices relating to cauls, glue, vacuum bags, and other elements of veneering a half-cylinder. December 6, 2011
Question
I need to veneer a half cylinder. Dimensions are 30"H x 30"W (so 15" radius). Back is flat, and will be veneered as well. Substrate is 3/4" poplar core ply with MDF face. Veneer is 10 mil paperbacked walnut burl. Plan to use my vacuum bag with Unibond 800 glue.
Question 1: How important is using a caul for the curved face? If it's important, what would be the best material to use?
Question 2: Will using my 4x8 poly bag stretch the bag to the point that it's unusable for flat work?
Forum Responses
(Veneer Forum)
From contributor J:
Almost all of my veneering experience is with thicker, shop-sawn veneers, so it may not be completely relevant.
In my experience, the caul isn't necessary for distributing pressure; the weight of the atmosphere is pretty damn even. It is important that the glue be thoroughly mixed; I usually mix once, let the glue sit for about 10 minutes, then mix again to eliminate lumps of catalyst powder. It's also important to spread the glue evenly, as if you were painting a wall - no lumpy squeeze-out from the edge of the roller is allowed.
Force distribution aside, if the veneer is cut a tad oversize, a caul can prevent the bag from crushing thin veneer around sharp corners. Your bag will be fine.
From contributor N:
I always use a caul, unless I'm pressing flat work face down in the press. I find for curved work, a piece of bending ply cut about 3/8-1/2" oversize works well. For a piece that tall, I'd put a smaller platen in the bag so you'll have more to work with.
From the original questioner:
Thanks. I'm planning to veneer the flat back of the half column first, trim it, then do the curved side. I'll trim the caul I used for the flat side just a tad undersize to use as a platform so the veneer on the curved side can overhang a bit. I'll take your advice on the bending ply and the smaller platen.
From contributor T:
Is this half cylinder hollow? The vac bag may crush it! For curved work, in a bag, I use a breather mesh product (available from veneersupplies.com). You can see the veneer through it and it's reusable.
From the original questioner:
The form will have 3/4" thick half circle plywood ribs inside. I'm thinking about 5 of them, including the top and bottom. So they'd be spaced about 5 or 6" apart. The mesh is an interesting idea. I'd forgotten about that stuff. I think I actually have some in the shop. Does it leave an imprint?
From contributor T:
The support ribs sound perfect. I have never noticed any impressions when using the mesh. I do like it when using an open grain, raw wood veneer. Any glue bleed will ball up in between the web and can be easily scraped off.
From contributor C:
If you place the mesh directly next to the veneer it can leave an imprint. Find something to go between the mesh and the veneer (besides the plastic that keeps the glue from sticking to everything). I skipped using a top caul once and just used the breather mesh. Fortunately it was the bottom (unseen) side of a project. The results were not pretty or smooth.
From the original questioner:
Thanks. I think I'll go with the wiggle board as my caul. Any particular brand? I have easy access to Kerfcore.
From contributor N:
I just did a top that was a 4" thick torsion box, with 3/4" ribs spaced about 4" apart. They telegraphed through two layers of 1/4" MDF! So I'd advise the ribs be closer together, and depending on what your curved substrate is, maybe reduce the pressure on your pump to avoid the scalloping that I got.
From the original questioner:
Yikes! I'm using a prefab form from Geometric Forms. It's pretty stout - 3/4" poplar core ply with MDF face. I'll do a test run in the bag to see if there's any deflection/distortion. I've thought about regulating the pump, but I wonder how much pressure is enough.
That sucks about your job. Did you have to toss it? Did you do a grid, or just ribs?
From contributor B:
This may be heresy to some of you guys out there, but what's wrong with contact cement - specifically 3M Fastbond 30 NF? It is, after all, paperback veneer here.
From the original questioner:
Now you've done it. The anti-contact cement mob is coming for you.
I'm not against using contact cement, but when I've done it in the past, I've always used phenolic backed veneer, or something like Oakwood bubble-free veneer. This is 10 mil paper backed. I'd worry about glue lumps telegraphing through the veneer.
From contributor B:
The bubble issue is why I like the 3M Fastbond. It is waterbase, and does not have that killer grab when you first touch the 2 parts together. It doesn't bond hard until you press it down. That gives enough play to allow the veneer to move around a bit as you smooth it down. Use a wooden smoothing tool - It is kind of like a wooden spatula 4" wide - never! a roller for wood veneer. Start from the center and work the tool outwards, kind of like hanging wallpaper. Fastbond, being waterbase, does not delaminate during finishing like solvent based contact cements will do.
From contributor C:
"Use a wooden smoothing tool..."
Is that anything like a veneering hammer?
From the original questioner:
You're only allowed to call it a veneer hammer if you're using hot hide glue. Everyone else has to call it a "smoothing tool" or "ice scraper."
I'm using a waterbased finish, so I'm not sure WB contact cement is a good idea. Besides, I'm committed to using my bag and Unibond. And I don't want the anti-contact cement mob coming after me. I'm surprised they haven't shown up on this thread yet...
From John Van Brussel, forum technical advisor:
There is an excellent video called Working in a Vacuum which goes into detail on how to veneer a cylinder in a vacuum press.
From the original questioner:
Thanks. If it's the one that Darryl Keil did, I have that video. I spoke to Darryl this morning, and he set me on the right path. The form is in the bag as we speak, pressing the flat back (the easy part). Looks good, so far. I'll be veneering the curved part tomorrow.
From contributor E:
For future reference, you can use bubble pack to reinforce a form. It's reusable and you can stuff it into almost any shape.
