Need Help with Lamination Warping

[Izzy Charo]

Hello,
I'm making a panel for a chest using shop sawn 1/8 walnut veneers, which I've glued using Unibond 800 to a 1/4 inch hardboard substrate. The dimensions are 17 inches by 14 inches. I veneered both sides and it came out of the vacuum bag flat.. Next day I found significant warp along the long dimension, and it won't fit in the dado. I have two questions: first, can I get this to be flat again, and second, what did I do wrong?
Thanks,
Izzy

[John TenEyck]

What did you do with it after taking it out of the vacuum bag? If you left it laying flat on your bench it would warp as the top would dry more than the bottom. (Unibond 800 has water in the liquid half of the product.) So it that's how it happened, take it and stand it on edge or lean it against a cabinet so air can circulate freely around all surfaces. It should flatten out again in a few days. A little careful application of heat with a heat gun to the crowned side will speed up the process, but stop before it's completely flat again.

OK, thought two. Did you orient the long grain of the veneer at 90 deg to that on the plywood? You should, especially on a really thin panel.

Beyond that, 1/8" is really thick for a 1/4" substrate. If the cupping wasn't a result of you leaving it flat on your bench then it's unlikely it's going to flatten out again, or stay flat even if you force it to do so. I normally use 1/16" shop sawn veneer for interior work and definitely would on a 1/4" substrate. However, you would be able to increase your substrate thickness to 3/8" if you reduce the veneer thickness to 1/16", a double benefit to staying flat. And don't forget to run the veneer cross wise to the grain on the plywood.

John

[Izzy Charo]

Thanks John. I did leave on the bench after taking it out of the bag; didn't realize that Unibond 800 has any water at all in it. I used 1/4 hardboard for the substrate, and ran the grain on the veneer along the 17 inch (long) dimension. I'll take your suggestion to stand it on end, with help from a heat gun, and hope it flattens. Appreciate the advice!
Izzy

[John TenEyck]

I hope it works out for you Izzy. Many folks don't realize Unibond 800 has water in the liquid half of the product. It's really no different than Weldwood Plastic Resin Glue, but with PRG you know there is water because you add it to make up the glue. But even if you had used epoxy to glue up a lamination you should never lay the piece flat on your bench. Same for wide panels of solid stock, too. Even a change in relative humidity will cause any panel to cup if left laying flat on the bench.

John

[Warren Lake]

whether it has water in the glue or not material will still likely bend. A friend was in a top shop and nothing was ever left uncovered over night. Even the lights in the shop have an affect on it. If you leave material flat on a bench solid whatever over night it should be covered.

[Bradley Gray]

I clamp work in progress parts/assemblies to a flat bench with stickers to allow airflow and limit movement.

I also strive to get finish on glue-ups as soon as possible.