Question
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.
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 hasn’t been softened).
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.
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.
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.
