Veneering Concave Surfaces

Tips and discussion on a tough veneering problem: inside curved surfaces. July 7, 2011

Does anyone have any experience with convex veneering? I have made two attempts with identical results using shop sawn 1/16" curly maple. My joints are pulling apart plus mid sheet cracking is occurring during pressing.

Forum Responses
(Veneer Forum)
From contributor J:
Those surfaces are concave, not convex. The curve you're working with looks pretty severe for 1/16" hard maple. Were you able to bend the veneer into place and hold it there by hand fairly easily, before attempting to glue it? If that would've been a struggle, your veneer was too thick to make the turn.

Did you apply glue to the substrate, the veneer or both? (Should be substrate only). There are significant sections of the joints between veneer pieces that are not taped. Did you remove the tape to take pictures, or were they un-taped during the glueup?

How quickly were you able to get it assembled, into the bag and sucked down? Hard maple reacts quickly and dramatically to moisture. If that (thick, rigid) veneer soaks up some moisture from the glue, it will quickly expand and curl away from the substrate. If it's rigid enough, the bag may not be able to force it back down.

On the (few) occasions when I've veneered a concave surface, it's always been over a hard male form, i.e. a convex form on the bottom, then veneer, glue and substrate. Without a form, it seems almost inevitable that the vac would clamp the highest parts on the curve (top and bottom of the leg) first, and that the veneer down inside the curve would not yet be tight against the substrate. This means that the top and bottom are already starting to stick, and that there's not enough veneer in between to reach the bottom of the curve. The veneer would be subject to a lot of stress as the press struggles to force it home.

From contributor R:
Because of the width of the panel the veneer should be seamed and cross-banded (Iíd use 1/28 maple as the x-band for color reason only) separately from the substrate. Then after this 2-ply assembly and core (I would recommend the ply core) has stabilized to the same mc, then apply and press it.

If the veneer is cracking open across the grain then you need thinner veneer or the veneer needs to be softened. On a concave bend this appears as a line, as the open crack is against the core. Veneer can be softened with water or steam. If the veneer is folding (compression failure looks like wrinkles) on the concave face then the only thing I know that works is thinner veneer (assuming the veneer hasnít been softened).

From the original questioner:
I stand corrected on the convex vs. concave. I applied the glue to the substrate and was approximately five minutes between the application and press time. The veneer is relatively stiff so I will try a thinner version. The joints were taped, I removed it to expose the joint.

From contributor Y:
I think what your problem is, is that the bag where it's trying to come together at the ends is stretching and pulling your veneer apart. That's the only logical reason I can see for this happening. Try either adding blocking to the ends of your panels or making sure you only have an 1/8" veneer overhang.

I'm curious why you're not using a clear poly or vinyl bag? Also when I do concave surfaces, I always pull a partial vacuum then take a roller and really roll the deepest part down hard, then finish pulling the vacuum and roll on it some more.

From contributor B:
Your problem is the thickness of your veneer. I recently veneered a concave built in bench and had the following problem you might want to avoid. I choose to use wacky wood attached to the frame with my face veneer contact cemented over it. I used a solvent based professional spray glue so please keep your hating on contact cement to a minimum. This is the industry standard.

Anyway, I ran short on veneer and had to splice it on the face. I was using mahogany veneer and even angled the splice. A few days later the joint crept open. Luckily this was paint grade and I just stuffed it with spackle. But, I bet it is visible over time with humidity changes. Maybe a press glue would creep less but in the future I will avoid face splices whenever possible.

From contributor J:
It's tough to keep the hating to a minimum when you admit it didn't work out so well a few lines later. Contact cement is the industry standard adhesive for plastic laminate-not wood veneer. I can't say I've never done it, but I've regretted it every time.

As the others said your veneer is too thick. I have actually had better luck on concave surfaces with no form or platen in the bag, just a bending ply caul rather than with form and veneer between the substrate. The method I like the best if it can work for your application is to veneer 1/8" bending ply flat in the bag, sand it, then press it to your curved substrate or ribbed form.