Balancing Issues when Veneering a Hollow-Core MDF Table
A table project involving MDF surfaces applied to a hollow torsion box and veneered raises complex questions about how to balance the assembly. October 19, 2013
Question
(WOODWEB Member):
I am veneering the center of a coffee table with a recon Macassar ebony 10 mil pb veneer (vacuum bagged with Better Bond cold press glue). The table construction is a 5 inch thick MDF torsion box with 1/2 in MDF skins and 1/2 thick MDF grid pieces at about 9" x 9" over a 4'x7' area. Or I can use 3/4 MDF for the top skin if necessary. Do I need to use a backer veneer on the top skin before I glue it down or is the Titebond III and brads used to fasten the top skin sufficient to hold the top fast and true? The rest of the table is finished with white 2k poly and the whole thing clear coated with 2k poly and then polished. I am concerned about the development of any warping or cracking of the finish. All suggestions for ensuring success are welcome.
Forum Responses
(Veneer Forum)
From contributor C:
Using 1/2" core you can get defections between the grids which will show sooner or later. Less so, but the same with 3/4 core. Your question is written in such a way that I sense you know you should balance the back. Balance the back.
From contributor P:
You might want to do a dry run in the vacuum bag and watch for deflections. You may not like what you see. Or you may try reducing the amount of vacuum.
From the original questioner:
Thanks for the response. I have read enough in the Knowledge Base to make a case for a backer veneer, but I am using reconstituted veneer and have no experience working with the product. I have no idea what type of expansion or contraction to expect. I guess I was thinking that MDF glued to a torsion box and well sealed with 2k poly would be in my favor for keeping the top stable over time. I prefer real wood veneer, but the client likes the consistency of the grain pattern on the recon veneer. Do you guys use reconstituted veneer? Does it have the same expansion/contraction characteristics of true wood veneer? I agree and didn't think about the deflection in the bag. I will surely use the 3/4" and do a dry run first with reduced Hg to see what happens. Still curious about the recon characteristics, though, if anyone has any input.
From contributor C:
We use recon all the time. Treat it like standard sliced veneer. At the very least, use a backer paper.
I didn't even consider the effects of the vac press during the press operation. We did a lot of experiments trying to vac bag curved shapes. I would not suggest building the torsion box then veneering unless you pressurize the box; if you don't, you should follow the suggestion the responder above makes.
From contributor M:
The thought of veneering the top before building the torsion box popped up for me... You may avoid the vacuum defection problem with this if it's not too late. Would like to hear your plan and the results.
From the original questioner:
Great suggestions! I think my only option is to veneer the top skin (both sides) then fasten it to the top, unless someone has a different idea. I didn't want to do it this way because then I have no mechanical fasteners in the center of the grid where the veneer is. Unless I finish nail through the veneer then use color matched filler for the holes (don't like this either). The other issue is matching the level of the veneer with the level of the finish on the border so that the clear 2k poly top coat is dead flat. I think I will make a test panel and record the finish schedule, then rout out the center for the veneer and match the surface level. Then measure the thickness of the finish so I know if I need to remove material from the center of the table top. Should have considered this when I designed the piece.
From contributor J:
Getting the veneered field dead flush with the surrounding border, which I presume is raw MDF? That's a tricky one. I don't have a solution for that at this late hour. Yes, you should balance that panel. I'd veneer it first, both sides - then attach to the torsion box grid. Not pinning the skin isn't an issue at all. I think the way to go is glue it with the face down in the press. If it's sitting on the platten, you're not going to get scalloping in between your ribs. You may get some on the underside then if you don't reduce the pressure, but that's easier to live with than on the top.
Maybe you want to calibrate your MDF panel undersize so it's the same thickness as the surrounding field after veneer?
From contributor P:
You could pocket hole the grid to the underside of the top so you have mechanical fasteners. I would only do this with 3/4 MDF. Be careful with the screws - they can telegraph through to the good side if you don't pre-drill.
Like someone else mentioned, your bigger challenge might be leveling the field of the top and the border. You can't do much sanding on that veneer before you burn through it. That polished finish will highlight any irregularity, of course.
From the original questioner:
I admit, I'm kind of stuck on this one. The table design is such that no matter what I do it will be a lot more work than I was planning on. Live and learn (way underbid this one). I will either exert more effort in veneering the whole table in the bag after finishing (after a test run in the bag) which requires me to build a brace (will explain in a second), or veneer the top before assembly. The second option will virtually guarantee that I will be spending far more time sanding and leveling the top coat because the finish thickness will be relatively unknown.
The table design is modern and is certainly unique. Imagine a pyramid which has been elongated (so the base is rectangular instead of square), then the top of the pyramid has been sheared off. Now flip the pyramid upside down and that is how the table sits on the floor (narrower base and wider top). The angles are such that the table is completely stable. Then cut out the center of the pyramid, leaving equal thickness on the top, bottom and sides. This thing has been a compound miter monster! The table is completely assembled except for the top.
I keep going back and forth on what to do but it's time to commit. I can't postpone working on the table any longer so I think I will make a brace for the center portion and throw it in the bag for a dry run with a low Hg. I really hope I don't end up wasting my time and having to veneer the top before assembly anyway. Should inverting the table so it is laying top down on the platen prevent surface distortion or scalloping? If it does distort under the platen, I won't be able to see it.
From the original questioner:
P.S. I just realized what you were saying, contributor J, about having a separate panel for the veneered portion and the border being a separate piece. I think it would be difficult to make the top look like one solid piece, but worthy of consideration. I suppose I could leave 1/16" of veneer overhanging all 4 edges of the panel, then set the panel in place and carefully trim to fit with an Exacto knife. Seems a bid dicey though. Anyone ever use this technique?
