I am creating a sunburst table 51″ in diameter with crotch mahogany I am not sure if I am going to make the core from plywood or not. The customer does not want an apron, do I need to veneer the bottom to prevent warping if so do I have to do the same pattern on the bottom?
Thanks for the help
Chris
Replies
Chris,
You need a backer veneer to prevent warping when veneering.
Keeping a veneered panel flat is an exercise in wood movement and the tension introduced while it dries.
Veneering introduces moisture during gluing which causes the wood veneer to expand. Depending on the core being used, it may expand some as well. The moisture content of the veneered item continues to decrease for a few days after the glue has bonded. While it dries, it shrinks. If you don't use a backer veneer, the panel will warp while it dries (it will become concave on the veneered side). Adding a backer veneer introduces similar moisture and tension to the opposite side keeping the panel flatter. The backer veneer should be of the same thickness and arranged in a similar grain pattern to the face veneer. I always use a backer veneer, but I usually order a secondary species of some sort, and it has always been good enough. I have even used shop made veneer (thick) with factory made veneer (thin), and it was fine.
Another thing that causes warping, even while using a backer veneer, is when the two sides dry at different rates. I had a panel warp once when I stacked a few panels after removing them from the veneer press. I used stickers between them. Bench - stickers - panel - stickers - panel - nothing. The top of the top panel was exposed to far more air than the rest of the panel surfaces. It dried more quickly, and it warped the panel concave on the top. I now stand panels on end while they cure for a few days making sure all faces have similar air contact area. Placing a piece of plywood on top of the previously mentioned stack would also make them dry at the same rate. The substrate for the panels mentioned was 1/2" Baltic Birch plywood.
To get the best results, you should make a sunburst backer veneer as well, so the movement of the top and bottom veneers is similar. If your core is going to be really thick plywood or MDF, you might get away without a backer. I wouldn't recommend a solid wood substrate unless you also make it a sunburst pattern as well.
Brad
Thank you so much! That is a big help!
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